Well, you can look through the PC-BSD web store to see the laptops that are known to work perfectly with PC-BSD. Another option would be to just find a nice one on NewEgg (my preferred hardware retailer) or tigerdirect, but be careful of a few things:
1) Make sure that it comes with a non-UEFI bios (so an older Windows 7 laptop) if you plan on doing some kind of dual-booting. The newer Win8 Laptops lockdown the bios so that you cannot have anything else installed unless you turn on the "legacy" mode (which de-activates booting into Windows 8).
2) Make sure that you have NVIDIA graphics support (non-optimus) if possible, although Intel works OK in some situations.
3) The wireless chipset is always the big unknown for laptops. But I suppose if you get a bad one that is cheaper, you could always make sure there is room in the budget to get one of those little USB wireless receivers that is known to work with FreeBSD and use that instead.
I know there are a few other forum threads down in the hardware section where a number of guys discuss different laptops, you might want to check in there and see if there are any others that are specially recommended.