Advice on a monitor upgrade deal

[dlong226]

So my brother is planning to buy a new desktop from dell and a 19in monitor is included with most of the deals. For 50 dollars extra, he can upgrade to the 20in (with much higher res, but not 1600x1200).

I have a 17in and was interested in paying the extra 50 to upgrade to the 20in, and then trade.

If i were to do so, on top of the 50 im paying for the upgrade, how much should I give my brother to switch my 17in for the new 20in? Assume the 17in and 19in he would have had have the same resolution?

Thanks