The price for upgrading your heroes increases a lot very soon. There are some interesting mechanics, too: reaching milestones (level 25, 50 and so on) gives your Heroes huge bonuses to DPS numbers, after you can afford that particular upgrade. However, getting there doesn't seem cost effective until you actually reach the upgrade.

For example, upgrading your first Hero to level 25 becomes extremely expensive and compared to the amount you'd have to pay for the second hero and DPS gains, it doesn't seem to make sense. But as soon as you hit level 25 and evolve your first Hero, their DPS will drastically increase and be well above the one of the second so all the money you've spent makes sense now.

So in order to upgrade heroes, work on them constantly, but focus on reaching the milestones, from the first hero to the last (first is cheaper and each star gain will make them much better than the more advanced heroes). Also keep an eye on health bars and other skills that might pop up during your upgrades.