A lot depends on how rich/poor you want your characters. In most of the implementations of this that I've seen, you set a conversion rating for gp to $. I've seen anywhere from 1 gp=$1 to 1gp = $20. Convert your gold pieces to their dollar equivalent and look it up on the d20 Modern chart p. 204. From there you can find the purchase DC of the item.
In the past, I've used 1gp=$10, and it has worked well. However, someone suggested I go with 1gp=$20 to keep characters from becoming too rich.
If you are trying to figure out a conversion rating for your game, pay attention to DC15 on the purchase table. Items with a purchase DC15 always cost the character one wealth point. In my game, it takes one wealth point to purchase a 1st level potion 50gp=$500=DC15.
If you set the conversion rating to 1gp=$20, then a 1st level scroll takes a wealth point 25gp=$500=DC15.
If you want your characters to be fantastically rich, then set the conversion to 1gp=$1. Assuming the characters have a wealth score of at least 15, they could buy any number of items worth 350gp (purchase DC14) or less without losing a wealth point.
If you are uncertain, I suggest you play stingy with a conversion of 1gp=$20 and adjust it as needed.