For perspective, we pay £1000's per day of penetration tests. We get a bunch of smart guys (from QuineteQ or KPMG) in and brief them fully on the architecture that we want testing. Then they go nuts on the application and the infrastructure. They're very good to us, but we do send a lot of business their way.
It depends what you want from a pen test - if you want genuine confidence that what you've got is built well and not open to obvious or even marginally subtle attacks, then you'll pay $$$. If you wnt to tick in the box for the ISO man, just get your neighbour to run nmap.