In Europe, it is possible for a 250 hour pilot to get the "command" experience required by logging P1 u/s. That is, the sector that is "theirs" they log it as "command". The captain has to sign it as well.
In the UK there is very little "GA" as we know it. Most go and instruct, a few go to Africa and so on. With the current downturn most low houred guys will probably have to wait a while to get their start.
The 500 hours multi crew time is laid out in the JAR requirements for an ATPL. That is the only way the costs can be reduced. If you don't have that, then you must do the 650 hours groundschool + 14 exams. Then you must do the JAR Instrument rating. 55 hours worth, I believe you can get a 5 hour reduction.
If you have the multi crew (prefer multi pilot type hours, much less hassle ie FAR/JAR 25 a/c) hours it can go 2 ways. If you are in current flyiing and you have 500 hours or more on the type you are going to do the check ride in, then all you need to do is the check ride (encompasses an ILS and NDB) and you have your ATPL. If you are not in current flying, then you must do a JAR Type rating (In the country that you did the theory) on a 2 pilot aircraft + check ride. Note, both check rides must have a CAA examiner on board.
So, in summary re costs.
Medical £400 or £189 if you have an ATPL
Note, must be done at Gatwick.
Exams £718
Course £3600 or there abouts
Living expenses budget £100/week (6 months)
IR approx £13,000 to £15,000
versus
Cheapest type rating I have found so far £8000.