HIPS do vary in price depending on the provider, searches required etc... it also costs more if it is a leasehold rather than a freehold as there is more information required and costs in setting it up. They are also Vatable so the VAT figure may be part of the additional cost.
It would be usual, if not paid up front, for your solicitors to pay it on completion with the agents fee so just make sure you get a proper breakdown in that the agents fee was calculated correctly (whether fixed fee or percentage based).
If you authorised the HIP to be produced then you are liable to pay for it. Unless you only agreed to pay a certain amount, preferably in writing, then it would be difficult to then not pay the full price.
Definitely make sure it was done and used in the sale but it is not unusual for you not to have a copy as it is really for the buyer and their solicitors.
The residential market has been suffering and I imagine they will not let this go easily as every penny counts at the moment.
If you're really struggling see if they will let you pay part now and part later. Best to front it and sort something out than risk them taking further action and the potential effect on your credit rating.