This is the important bit, and I'd be happy for us to overpay (i.e. >20m for 4 years), because I don't believe spending that money prevents us doing something else with it:
Payroll should not be restricted to less than $140-150m and could possibly be a lot higher;
If we want to extend Donaldson's contract next year it should be affordable with a bit of flexibility even with Eddie signed;
Free agency is weak this year, if both Eddie and Jose left we would struggle to get adequate replacements;
A trade for a decent replacement for either would be very difficult without either losing much needed prospects or weakening an already depleted starting group, or taking on an even worse contract (Votto for example);
If the worst case scenario happens and we suck next year (which I don't believe will be the case) Tulo and Martin should be movable to contenders with money to burn as both are considered good players and born winners;
The alternative is likely to be a combination of a bargain basement free agent or two, a prospect, a flyer on a waiver claim, and/or Smoak, all of which increases the chance of us taking a step back.
If someone completely blows our offer out of the water, don't go crazy, but this is definitely a situation where our only priority should not be getting perceived value for the contract.