Hi Chris,
I am not an expert, but I will try to help you with your question. A few years back I tried doing the same thing that you are doing, a team set with as many different backs I could get/afford. My team was Cleveland, you know the team with 3 different Cy Young cards and different Lajoie cards.
Need I say more!
Your question is a complex one because there are so many factors involved here such as:
availability
front/back combinations
other player/team specific collectors
bidding wars
So trying to throw out estimates is can be difficult
Here is an example for you, last night on Ebay I was going to bid on a T206 of Al Burch (batting). The card was raw and probably would have got a grade of 3/40, but this Burch version always, from my observations, tends to go for a little more than a regular common T206. This Burch also had a Sovereign 150 back. Final closing price was $51. Needless to say that I didn't win this one.
What got me away from doing my Cleveland set was when I saw a Bill Bradley (portrait) with a Hindu (brown) back go for over $300 on Ebay. I knew that I wasn't going to reach my goal.
In no way am I trying to discourage you, but keep in mind, this kind of project is a
long-term project . At first the cards will come quick, but sooner or later the brakes will hit. Unless that Nigerian lottery check finally comes in.
I hope that I answered your question and good luck if you continue with this. Also follow the advice in the posts above mine. Their suggestions will help you greatly.
Jantz