Street prices are usually achieved by adding junk (typically an item with 0 street value) into the trades or by trading items with equivalent street prices.
Example:
Item A is worth 100m on GE but 230m street.
Item B is worth 10m but 50m street.
You'd have to pay 230m for Item A + extra junk provided by seller.
You'd have to pay 50m total for item B + extra junk provided by seller.
You could also pay with enough total "extra" street values instead of pure cash. Exemplified:
You give some of item B to add "street value" to your side of the trade. Then you use the item B + some cash to balance the market value. You buy item A + some junk to balance the street value.