Speeding up progression a bit is surely a good idea. If you look at the price increases between upgrades especially at the higher end it becomes disheartening (e.g. Master Wiz->Zap Dmg).
The question is what you want - should the player be more looking to upgrade the current "life" (then lower the costs a bunch) or rather rebirth often and upgrade the whole run (which seems to be the standard with games that have a rebirth mechanic)?
You are the god of the game, you make the rules :) I'm sure you've played a fair share of idlers before deciding creating one...so look at what made those games fun and take inspiration from that - if you want. :)