The price of a multi-gene morph is generally always more than the sum price of the individual morphs. The individual components cannot generally be added up to get a price for a multi-gene morph. Lower supply is one reason for the higher price but also the time involved (which is also a factor for the lower supply).
For a more extreme example, I saw a pied albino the other day for close to 15k. Individually, a pied and albino are worth around 1.2k together. The years and patience to produce a double recessive morph are represented in the price.
$750 is a good price... if she looks good, you should snap her up!