I would suggest posting your questions on the Fodors.com Africa talk board. There's a very active group of users there with some expert advice on African safaris. They were enormously helpful in planning my safari.

For Kenya, I would think that end of July-August is the best time as the Wildebeest Migration is in the Mara at that time. I can't help you much with an itinerary as I've planned and booked for Tanzania, so my research is concentrated there. I would say that it's getting very late in the season and that much of Tanzania is fully booked by now for next summer.

As for safari costs, I'm paying $2825 per person for a 4-person, 10 day safari along the Tanzanian northern safari circuit. This is a private, customized safari for my family with our own driver/guide and vehicle. This includes everything (accomodations, driver/guide, vehicle, meals, internal air transfer, park fees, etc...) except incidentals like souvenirs, insurance, laundry, etc... The price goes up/down dependent upon the size of your party. We're staying at a mix of accommodations - including high-end, luxury tented lodges, Hemmingway-style camping, and simple hotel-like lodges.

Airfare to Nairobi shouldn't be too much. I'm also flying from SFO, but going to JRO. What my family is doing is using award flights to get us to London from SFO, and then purchasing a LHR-JRO RT ticket for about $1000. We'll put in a couple of days in London to break up the trip a bit. AMS also makes a good stopping point.