But there's not much to do with gold that's worth spending it on besides retinues. Retinues aren't cheap, but if you're at war continuously you'd end up with a bunch of very unhappy vassals (because of the raised levies penalty). So you tell me what's cheaper... and add to that that retinues is the only way to get a high number high quality troops (knights and heavies), which can save you a lot of casualties compared to regular levies.
If retinues are too costly, try lowering the reinforce speed. Or perhaps your retinue is too large, hard to tell from here