See page 3 here for the conversion requirements:
LASORS Section E. Check out the FTOs for the hourly dual multi IR rate and sim cost and work out the costs involved, probably around £3-4k.
Then remember you'll need about two hours solo hire for the test plus the bargain £578 examiner fee. Oh, and not to mention £276 for the licence issue
One downside of going the FAA route is getting up to speed on the FAA regulations... the written test isn't so bad, but you really have to know your stuff for the hour or so oral interview with the examiner before the flight test. If you do some hour building before the IR in the same country, this will help you get used to the local rules.
See
The FARs for the FAA IR requirements (40hrs instrument to include 15hrs with an FAA instructor). Search under "Current FAR by Part" for Part 61, then look for Section 61.65.
Take a look at South Africa too for the IR; from what I hear it's cheaper than the US.
As for what employers think of it, your JAA licence will be the same as one gained from a £60k integrated course. The only way anyone can see your history is by checking your logbook, at which point you can ramble on about your experiences in different countries make you a more rounded pilot... after all, there aren't many airline pilots in UK who don't regularly fly abroad
