It depends on the brand, you can send it back to the manufacturer. Or you can use a reputable shop that works on all brands. IMHO the best out there is Radio South. [link]http://www.radiosouthrc.com/[/link]
On the pricing here is what their website lists
FREQUENCY CONVERSIONS
The FCC requirements for "gold" is -55db sidebands at +/- 20Khz. An additional requirement is the frequency accuracy must be within +/- 1.5 KHz from center frequency. Most transmitters can be "gold stickered" at a cost of $15.00. Pro-Line systems are $40.00, due to special components needed to update them. Kraft systems are $50.00 for pre-1976 transmitters, and $25.00 for 1976 and later versions. Frequency changes are common in order to obtain a frequency that is less crowded or to move away from known interference at a local flying field. Costs vary from system to system, due to the price of crystals required for the change. Most newer systems use dual conversion crystals in the receivers. Prices vary, but most Dual Conversion systems are $49.50 (TX and RX, including tuning) to change. Single conversion systems are usually $39.50.
Since you already have the crystal I would guess it would cost less. My recommendation is to call them and ask the actual cost.
Ken