Sizing down is easy.
Sizing up is a lot more labor intensive, plus you have the cost of the additional gold. The piece you put in has to be thicker and wider as well.
Then is get filed, sanded polished etc, to match the existig piece, and what was shank material now is dust/filings.
If the band has side stones, all the more difficult, as sometimes those need tightening in their settings.
Additionally, like most services, the quality of the workmanship also enters in to pricing and time to do it.
That price doesn''t sound to outrageous, especially if the quality is well done.
Rockdoc