Assuming that they are innocent (I think that they are), what do you expect them to do?
¨Ah **** it, this is costing too much, lets just let her go and get on with life¨ ?
She is (was) their daughter and they have demonstrated very clearly that they are not the moving on types, they will keep looking for the rest of their lives or until some DNA matched bones are found (very unlikely).
To make matters harder, the Police are not closing the book on it either, which is unusual, granted, but if the old bill are saying that they are still following up things then nobody can really tell them to stop with a clear conscious. The country may be broke but look at it this way, the Big Ben refurb is currently quoted to cost 61 Million, Houses of parliament refurb 5.7 Billion. When we are throwing money around like that then a few hundred grand to try and find a little girl is trivial.
If wasting money is the problem that people have with this then I would suggest that there is other, more obvious low hanging fruit to rant about.