Recent XLE sales in the Toyota Sienna Prices Paid and Buying Experience discussion in the Edmunds Forum are closer to $35,000. Don't know if those are the FWD versions.
Try appraising it and see what True Market Value is for your zip code, color and options. When I try that with a Vegas zip code, no options, I come up with $36,610 TMV and that's before the $1,000 customer cash incentive that's ending today (1/7/2013).
If you are as far apart as it sounds like, I think I'd check with some other dealers.
There are a couple at Fitzmall.com for ~$37,300 for another point of comparison.