I think you're looking for the adjective exorbitant:

(of a price or amount charged) unreasonably high.

‘some hotels charge exorbitant rates for phone calls’

And yes, it specifically means unreasonable, but not necessarily unaffordable. In the example sentence I've quoted from the Oxford dictionary, a hotel guest can probably afford to pay a high rate for a phone call, but they find the amount unreasonably high.

Editing my original answer, since it seems that I misunderstood your question.

Unreasonable by itself is probably not precise enough since it fails to differentiate between situations where the price is intrinsically unreasonable (no one should be expected to pay that price) and ones where it's unreasonable for you to make this purchase at this time (because there are wiser uses of your money). In fact, this ambiguity is what led me to misunderstand your question.

If the price is intrinsically unreasonable, then the price is exorbitant.

If the price is reasonable, but it's unreasonable for you to make this purchase at this time, then here are a couple of options:

The purchase is frivolous or the item is a frivolity. This would not often be used for an expensive item like a house, unless you are rich enough that buying the house has very little impact on your financial security. It is more often used for smaller purchases like new clothing that you don't actually need.

Making the purchase could be imprudent (or "not prudent"), meaning lacking in good judgment. That is, the price might be a fair price for the purchase, but it shows lack of judgment for you to make this purchase. For example:

I love that house that's for sale down the street. I could certainly afford it, but it would be imprudent for me to buy it now when I should be saving up for my children's college education.