Without the OP specifying which smartphone it is, I really don’t see how anybody could possibly advise regarding the cost of replacement.
It’s a bit like asking how much one should expect to pay for a replacement engine for their car.
Just to give a couple of examples, if it’s a generic Chinese handset, £20 or £30 may well be more than the phone’s worth, but if it’s an iPhone X, a 3 figure sum to have the screen replaced at an apple store would undoubtedly be money well spent, as attempting to replace the screen with a cheap counterfeit would at best significantly reduce the value of the phone, and at worst also render it unusable.