Requesting a breakdown of costs seems a reasonable request.
In contracts, there are 2 types of damages- either penalty clauses, or liquidated damages. Liquidated damages are a 'genuine estimation at the loss of one party'. These clauses, which are generally calculated on a case by case basis, rather than imposing a set amount, are enforceable in contracts. In some cases though, there are easily quantifiable amounts that can be preset. For example, a mobile phone company would know that if you cancel your £20, 12 month contract after 6 months, liquidated damages would be £120 (the remaining 6 months x £20).
The other type of damages, penalty clauses, are not enforceable in contracts. This is the crux of the major bank charges case that's in the High Court as we speak. If the banks cannot prove that their standard £30 fee for being overdrawn is a genuine assessment of loss (rather than an arbitrary amount), then they cannot enforce them and many people will be claiming refunds. Also, you'll notice that newer signs warning about clamping charges include a breakdown of costs (clamping, towing and storage etc.), rather than a (seemingly random) lump sum. This is to make their charges more likely reasonable and thus enforceable.
I say this as it may be that the £250 is a penalty clause that is unenforceable. However, the estate agents may know from experience that they incur this amount, so it could go either way. Without seeing the contract and a breakdown of figures, it's impossible to say. Buy if I were you, I'd ask for the itemised invoice, and then take it from there.