If you are going to spend both money and gold and you have a bit of free time, there are some ways to drop this price by a lot.
(1A) Game the quest system
There are three 200 gold quests, three 300 gold quests, one 600 gold quest, and one 800 gold quest. Fortunately, those three 200 gold quests are 'universe' quests (i.e., play two games as a Diablo, WoW, or SC character). The trick is to just not complete two of the universes, and they get taken out of the quest pool.
Currently, because WoW is the only universe with all four roles, this means you have to get or have access to one WoW character of each type and limit yourself to playing them--at least until they come out with a Diablo support or SC warrior.
The last point about this method is that it does requiring playing everyday. I have more time than some, but not as much as others. This means that sometimes I need to do cooperative to burn through the 'Play 8 games for 800 gold'. Given that I can usually get through 8 coop games in 8-10 minutes a piece, this is still a much more lucrative option. But again, not for everyone.
**(1B) Gazlowe is great for coop to game the quest system.
You should do quickmatch for gold if possible, but if you have to do coop because of the time constraints involved in gaming the quest system, then Gazlowe is great.
Pretty much no matter how bad your team is, you can create enough distractions for the bots that he can win the game. You can solo a lane early on, and if you do the turret-boss-killing build, you can solo pull siegers and bruisers at level 4 and bosses at level 10. With all the mercs heading down different lanes, you can usually join the busiest lane with your robo damage and clear out all buildings up to the core. You just do that one lane at a time until all minions and mercs are going on core and even if your team isn't that good, you've won it for them.
You'll burn through lengthy quests and get the gold very quickly with this strategy.
**(2) Wait until heroes go on sale to buy them with real money.
Hero prices are between $4 and $5 (sometimes as low as $3.79) when they go on sale. If you don't care about the skins or mounts in a bundle, this is a much better deal.
(3) Save your gold for the expensive heroes
The 10,000 gold heroes rarely go on sale, so use your gold for them.
Conclusion
Just by doing the above, I've been playing for a little over one month, I have 23 of the 35 heroes, soon to be 24, and I've spent around $44. (Granted, that free one-week stim pack compounded by the 25% XP increase helped get all but two heroes in the game to level 5 pretty quickly.)