The migration is in Kenya's Masai Mara.
The Crater is in Tanzania.
The countries aren't as close as they appear on a map.
If visiting both you require Visas for both, US$50 for Kenya, US$100 for Tanzania along with Yellow Fever inoculation and proof of same for entry to/from/between countries.
July/August are peak season both countries with prices to match, but reasonalbly priced lodges/tent camps can be found.
You also have to consider whether you wish to drive or fly between areas within Kenya. Driving isn't always less expensive than flying... this can vary.
But in reality... what is your estimated budget?
During this period you have to expect to pay DAILY PER PERSON about $350 (if lucky*) to 650/person, to include: room/tent, meals, park fees, game drives, guide/vehicle, flights as applicable and taxes (of course.).
*guide/vehicle per day is approx. $225; park fees range from $50-100/person; so what's left has to be for a place to lay your head!
There are some Tour operators out of the States that do cover both countries, but know that these do entail lots of transit time on some nasty roads. Have you checked the likes of Overseas Adventure Travel (OAT) who also include int'l air. There are other such companies.
As to air from the West Coast - well, there's Delta via AMSterdam; also recently Emirates started flying from the West Coast and often has competitive fares. But, again... flight costs during these months don't come cheap. Best to check a site as Kayak www.kayak.com for schedules/prices. My guess, you have to figure minimum $2K (again if lucky).
In country outfitters to consider:
Eastern & Southern Safaris
The Safari Company
also check the KATO site for further names www.kato.com (or is it www.kato.org)?
You've got some work ahead of you on such short notice. Good luck!