I think the rules are explicit that there is no maximum bid, other than it being illegal to bid more than you have.
I think it's odd that there's no designated penalty for bluffing, given that it might even happen accidentally. Having to pay all your gold and not get the god's favor anyhow seems suitably bad ... unless you really didn't have much gold at all. Maybe you should also forfeit your next turn's income.
Keep in mind that because of priests "more than you have" and "more than you can pay" may be different. You totally can bid more than you have, as long as you can still pay your bid thanks to your priest discount.
And to circumvent the problem of someone accidentally bidding to much money, we just play with open money. Keeping the money hidden is pointless anyways, as it's non-secret information in the first place, so that just makes it easier for everyone.