When looking for car rental prices I typically look up a travel site (ie: yahoo travel) to find the car rental companies in the area that I need. I then search out rental codes for all of those agencies. Sometimes you can get pretty decent rates from those codes. Plus compare those to what's on the car rental companies sites.
In my experience I have found that minivans are one of the most expensive vehicles to rent, do you ahve to have one? Can you go with a full sized sedan instead? I have found that SUVs are also cheaper than minivans. For an upcoming trip DH and I are renting two mid sized cars for our group (6 of us) as with the codes I found it comes in about $350 cheaper than renting 1 minivan. That's just an idea.