Getting one on the cheap is going to be tough, everything involving displayport is expensive.
Cheapest one I came across was 33.99 + 5.99 s&h
http://www.newegg.com/Product/Product.aspx?Item=N82E16812200458
On a side note, do you know if you can run a vga > dvi connector through your dp > vga so you get
dp > vga > dvi
and still have it operational? I'm sure it wouldn't upscale back to digital, but if you only have dvi connectors it's a viable solution for those of use that can't find the dp>dvi dongles in stock.