Main ways to get gold in game: Quests
Jona's Quests; reset mostly with arenas; 16 for 2g apiece=32g each reset
Arenas; reset normally once a week, but less lately
; 5g-50g each; total of 130g each reset
One arena you forgot was Deadfields Arena. Ranked between Bleakwoods Arena and Bellroot Arena, it is advised to have a party of 150+ for it. Battles from floor 5 to floor 7 start giving good experience, ranging up to 10k for a couple fights. Upon completion it will give 22k xp and 35g.
Fatal Fortress (WHA) is 8260 XP and 7773 silver for soloing all battles and 5g upon completion.
A team of 25-35+'s is suitable. Very often alted.
A Hero is Born (SVA) is 61 827 XP and 33 581 silver for soloing all battles and 15g upon completion.
A team of 75+'s is suitable. Often alted for most of it.
Chilling Treasure (BWA) is 104 730 XP and 87 435 silver for soloing all battles and 25g upon completion.
A team of 100-125+'s is suitable. First part is often alted.
Stolen Pumpkin Seeds (DFA) is 190 577 XP and 92 037 silver for soloing all battles and 22k XP/35g upon completion. A team of level 150+'s is suitable. The enemies here at the beginning are quite weak. Only start to get strong at floor 5. Beware the 6 Strawmen all hitting 3k each on Floor 5 Room 5. I use it to tell where I am
Beginning floors often alted.
Badgers and Spiders and Bears, oh my! (BRA) is 324 504 XP and 137 722 silver for soloing all battles (258 195 XP for duoing all battles) and 35k XP/50g upon completion. A team that hits 30k+ is suitable.
Bear Invasion 2 (CWA) is 160 498 XP for soloing all battles according to the wiki. It is one of the only two arenas to have 8 rooms, rather than 7 floors with 7 rooms each. It is the third hardest arena. The bears from Bear Invasion will all be in here. Upon completion, 100k XP, 50g, and a Bear Head (no stat bonuses) is awarded. A team of 300+'s should fare farely well, but will require quite a few people.
Underworld Invasion! (UWA) is the second hardest of the arenas. A big team of 300+'s is necessary. It is one of the only two arenas consisting of 8 rooms, rather than 7 floors with 7 rooms each. Enemies are powered up versions of normal UW monsters. The Water Elementals and Demon Pirates have 3 500 000 HP, rather than 30k and 40k respectively. Rewards upon completion are 300k XP and 250g.
A Plagued Arena (PGA) has been beaten only a few times, including with 5 members of Glutamine Lovers (check the top guilds page). It is extremely hard and gives 600k XP upon completion. It is considerably the best place to train while hitting 50k+ damage. It gives no gold, but is worth mentioning.
Bear Invasion isn't too hard for a few good people to complete. An Evergreen Soldier's Catapult V helps a lot, doing about 40-50k damage to one bear. All bears have 1 000 000 HP. Two are attack based (the Iron Bears) and two are magic based (the Forest Bears). They hit me (100 def and 87 or so res I believe) for about 5.6k damage or 11k crit. A few Snow Bandits with Frost Daggers 3 will make pretty short work of them though. The reward, as said is 100k XP and a very easy way to level from 1-40, or above as necessary, as well as a Bear Claw Lance which becomes pretty useless. Easier versions of these bears with 250k HP will be seen in BRA. These bears themselves appear in BI2.
The last UW quest, Demon Pirate is really easy and gives a one-time 100g.
The Plagued Grounds Quests all give really good gold and XP rewards. Note a fair sized team and an anchor are recommended. Here they are:
A Deadly Bat-Tie: Three 1-room shacks with about 7 battles each. Rewards are 130k XP and 60g.
Lost Diamonds: Two 1-room shacks with several battles each. Pretty easy for 35k XP and 30g.
Shacks of Evil: One 2-room shack with a decent number of battles each. Not long for 60k XP and 20g.
The Creeps Beneath: One 3-room shack. Easy other than the middle floor. The battle right beside the trapdoor has a 755k Troll Pirate. It gives 100k XP and 70g for completing it.
A Tunnel of Regret: 7 room tunnel with pretty easy fights. Last room has stairs. Won't take too long and gives 210k XP and 130g.
A Key Worth Gold: This is the Game's only Hidden Quest. Go to the top left square in PG, straight up from the start and you'll see a dungeon hole, but no entrance. Click to just the top right of it and you'll go in. A few battles and you get 180k XP and 60g. I love hidden quests
A Sacred Disruption: This is the only really annoying quest in PG. There are 8 rooms with abstract pathways between them. After a long while, you'll receive a hefty 200k XP and 200g. I liked Tunnel of Regret Better!
That's the end of the Plagued Grounds Quests. The bat battles give horrid XP and the others give about the same as the UW. Total quest XP comes to 915k
Total gold: 470
Last quests for gold are Rosie's Map's 1-5. 1 is the only annoying one, half of Fatal Fortress. Floor 3, Room 6, right battle to be exact and there's your drowned! Recommend doing it when FF resets unless you're like me alting it all the time for people. 2g apiece = 10g from Rosie! One-time only offer folks!
Also worth mentioning, this time for actual gold is referring people. You can easily train them to 10, can't you? There's 20g right there! Take them through BI, UW and PG and they'll almost be 100. A bit of powerleveling and they'll be there for an additional 80g! I'm not sure if it's allowed for your friends to give you their account if they don't like the game and then you can train it with 2 tabs or computers.
One last way of earning gold is using Kongregate Kreds. You can only get them for free if you develop a game good enough to have a tip jar on it so don't ask! If you're willing to pay, you can buy them.