Costs can range all over the place. It's easier to figure out what you can/want to afford and then try to create an itinerary to stick within that budget.

What are your interests beyond safari (how long on safari vs in cities)? Are you only interested in Kenya for safari or possibly other areas as well (such as Tanzania)? Are you traveling alone, as part of a couple or as part of a group? Are you willing to share a camp vehicle on safari or do you want your own vehicle? Do you wish to fly to different locations in Kenya or drive (the roads can be awful and take 5+ hours to go from one camp to another)? What time of year do you wish to visit? You say you wish to interact with locals... in what way (mission trip to build a school or just general interactions while sightseeing)? Do you speak the language? Do you want a guide/driver?

What draws you to Kenya in the first place and why 21 days? Are you willing to shorten your days to adjust for budget? What are your lodging standards/requirements?

Do you want to do your own planning and making arrangements?

There are 1,000 questions to answer before people can really try to help you. I'd suggest going to your library or book store and starting research in the travel guides, reading through some of the trip reports here and putting together a better idea of what you want before asking for ideas of cost.

Good luck!

Thanks that was very helpful.
Is it more cost effective to purchase a tour prior to arriving or once you get there?

I wouldn't go without arrangements already made, certainly not for the length of time you're looking at.