Hope this helps...
From my understanding, as a Brit with FAA Commercial Multi-Engine Instrument (& JAA PPL, FAA CFI/II)...
NB: There is no conversion in the same sense as FAA to Transport Canada (a relatively simple process with NO flight test, and just watered down "written" exams for Commercial, Instrument or ATP)
To get to the same licensing level in JAA-land; ie, CPL/MEIR (fATPL) I would need to do the following, as a bare minimum...
Those stupid 14 exams (or only 12 if I didn't intend to get an ATPL, just CPL/IR). I have heard rumors that I could be "signed off" by an approved FTO instead of having to prove 650 hours of ground-school and taking their course. eg, They could approve me to the CAA as being ready to take the exams (as I already hold ICAO qualifications), and I'd just have to tip up at Gatwick and have at 'em, then re-sit whatever I fail! I doubt any FTO would waive their "right" to sting me for upwards of 1000GBP, so I'd likely be signing up to Bristol.gs and doing that. So meeting ALL of the theoretical requirement is a MUST. It needs to be done.
As far as the flight training...
I would need 15 hours minimum instrument training (which I have from doing my IMC) with a JAA instructor (presumably in a G-reg), plus whatever training I require in preparation for the CPL and MEIR flight tests. All that, plus a class one medical.
So, best case, maybe 1-4 hours flying on top of that, in preparation to take the UK skills tests. I have it on good authority that the UK JAA CPL standards are not as difficult as the FAA, and that the IR is more of an ADF rating... Minus the NDB work, the IR is similar in scope and the skill level required, to the FAA instrument. Plus there are no oral exams for JAA, probably because you've shown that you know at least 75% of what you should by passing the written exams.
As far as costs... very cheap in the US in comparison to Europe, even with the current exchange rates and economic situation. Just don't forget about the cost of; flights to/from the US, places to stay, means of transport, travel insurance, food etc. Those would be the negative points. On the plus side, you would save on the flight training costs, the weather can be more favorable (especially here in AZ), and you might get lucky and train at a good flight school. (I have heard rumors there are some schools that aren't too great, especially out in FL) Another good thing, is no snotty-ness, or class system here in the states.
After all that, I can't give you a cost! But you should be able to work it out for yourself from what's been said.