We just did this and the cost will depend on the size of the cat and the dimensions of their carrier (which must be at least one size larger than the carrier you'd use to take them to the vet).
We have a 20lb cat Max. He's a big fellow and this increased our freight cost.
We paid about $180 for microchip insertion and rabies, another $150 for the USDA vet inspection (7 days before flying), then about another $150 to UPS overnight the USDA and Annex IV forms to the USDA headquarters in Albany NY for stamping.
The freight cost from Boston to Heathrow on Virgin was $900 (Virgin makes you use exceptionally large carriers - I think it's a bit of a money making scam posing as animal welfare). On arrival in the UK you have to pay another approximately $500 processing fee.
So, a la carte for one cat this comes to nearly $2000 per cat. If your cat is smaller you might be nearer $1500 per cat. The cost for multiple cats is slightly reduced since they can all go on the same annex IV (so the mailing to/from USDA and the USDA fees are reduced). The UK fees are much reduced for multiple animals. I'd guess two cats might run around $3000-$3500 all in.
If moving in September start researching now. Research carrier sizes and call airlines for quotes (I doubt you'll be under $500 per cat, and probably closer to $800). Virgin doesn't charge you until the day you fly because they know your cat might not get on the plane if the vets find a problem, if the paperwork has a problem, or if they don't like your carrier. This is good because you can book the flight well in advance and then get on with planning toward it without a financial penalty (which was good for us as the vet advised us to leave our other elderly cat with a relative, which we did).
Also find a USDA accredited vet who can perform the pre-flight health checks within 10 days of the flight (no earlier). Around Boston very very few vets are USDA accredited, and we had to book a month in advance. Also find out where the USDA office is to which you have to send the USDA and EU forms to be stamped - it might be out of state so you'll need two UPS/FedEx overnight airbills - one to send, the other to return. You can email your local USDA office to ask them for advice. Our USDA accredited vet was in downtown Boston, but the forms had to be sent to Albany NY USDA for stamping (ugh bureaucracy !)
Forms you'll need:
Rabies certfificate (See "Vaccination certificate" here
https://www.gov.uk/take-pet-abroad/rabies-vaccination-boosters-and-blood-tests for requirements)
Annex IV - one form can cover multiple cats and is completed by USDA accredited vet
http://ec.europa.eu/food/animal/liveanimals/pets/docs/reg_577_2013_part3_annex4_en.pdfIf your cats don't have 15 digit ISO compliant microchips (which are unusual in the USA) then check with your UK port of entry that they can read the chips - or rent a chip reader. We entered through Heathrow and I have to say that the Heathrow Animal Reception center people were friendly and helpful throughout: and they can read a lot of different chips, not just the ISO ones.