When I was there in February I stayed at Little Governors Camp and did the balloon safari with them (it takes off from behind the camp). As it was during the post election violence and there were very few tourists we were able to get the balloon flight half price $200 per person rarther than $400 per person. We booked direct with the camp on our arrival. The flight lasted about 50 minutes, on landing we had a champagne and full cooked breakfast beautifully set out under a tree and then about a 2 hour game drive back to camp. We had to abort our first landing because we flushed a cheetah out of the grass as we were coming in!
I think there are at least two other places in the Mara (Fig Tree Camp plus AN Other) where balloons take off from, all are a similar price, but Governors has been doing it longest, has a good safety record, and was the most convenient for us. In full season they operate 4 balloons per day. They do accept non resident guests for the flights, you pay the balloon flight price plus a transfer fee to the camp you are staying at who will drop you off at Governors at 5.30am and then meet the balloon where you land and take you back to your camp.
Depending on where you are staying in the Mara, one of the other balloon take off sites may be closer and due to the early start that usually dictates who you book with.