I would flat out reject the offer, you are getting nothing with Holt and Caddy, and even though Schaub is a little bit of an upgrade over Favre you are not getting enough out of the trade to offset the loses of Thomas Jones alone. Jamal Charles is going to be the start the way it looks from here on out, for what that is worth with KC and Hester has clearly been better than Holt this year. If you are really concerned about you QB position there are number of them out their this year i.e. Brady, Peyton, Rivers, Brees, Rothlisburger, Palmer, Favre, Schaub, Romo (and even possibly Hasselbeck now that he should be back to full speed). You are not really hurting at QB as bad as you think and giving up Thomas Jones would be huge, especially now that Leon Washington is out for the season thereby putting even a large load on Jones.
Without know both your rosters, I cant even recommend a counter offer.
I happen to have Peyton Manning and Matt Schaub on my roster in a 10 team keeper league. I have been trying to trade Schaub off now that Peyton is past his by week, with little success and very weak offers. Everyone in the league knows I will have to drop him at the end of the year and just think that they will draft him in the first round. One offer I made was for me to give up Schaub, Bradshaw and either Houshmenzada or Manningham, and my #3 pick in next years draft for Andrea Johnson and either McNabb or Sanchez and the guy has rejected it. But then there is a reason his team is one of the worst in the league.