Another option would be to have diminishing returns with higher levels of gold, so that once you have enough, you can spare 500 or 1000 and only see a small dropoff in your statboost, ostensibly more than offset by the stat gains of the item you purchased.
Dunno if you play Civ4: Fall From Heaven 2 mod, but the dwarven civ there gets a civ-wide boost based on how much gold they have built up in their treasury (and below a certain baseline, it actually gives you a civ-wide penalty). This is divided by the number of cities they have. End result is that you DON'T want to be expansionist, at least until you've got enough gold that dividing it would still leave with enough to qualify for at least baseline if not boost.
Of course, as you grow, it paradoxically becomes easier to grow. If you have 10 cities, adding an 11th doesn't hurt you nearly as much as adding that first one.
So with that in mind....I dunno. Maybe gold divided by level or number of items or something. You'd have to tweak the balance carefully because you don't want to hamstring a player for buying things like boots that they really need. If it's a single stat gain (say armor, or health), it probably wouldn't be worth it to hoard for that long. If it was a bigger gain (like a permanent scaling steroid), it could be gamebreaking.
One nice side effect is that the runes and masteries you'd want to build would be totally different (and you'd have to take that into account too...a page full of gold gain runes with a mastery page with all those "increased gold from kills/higher gold accrual rate/higher starting gold" choices could start you off seriously OP early-game.
Maybe it could be a single-stat gain, but a more potent stat like cooldown reduction or EXP gain or something.