Best thing to do is put them in a pair of pantyhose with their head turned back and make sure all of the feathers are laid down smooth and you should be able to keep them frozen that way until you are ready to mount. Prices should be between $100 and $200 depending on the mount. This is how I do it but im sure you will get many different answers on here.
In my experience, complete opposite...
Most taxidermist will tell you not to put it in pantyhose. Don't vacuum seal it either.
Double bag it in Freezer bags. and they last for a couple years before getting mounted. You can wrap the feet in something to keep from freezer burn, but I am not 100% on what that is.
Expect to spend $225+ for a quality mount. There are lots of folks who do birds for less, but their work isn't "quality" in my opinion. Look at folks' birds before sending your trophy to them. If you like there work, and they are "cheaper" that is your call.
Also, if you have a service/expertise you can offer, try offering barter deals. I've had good luck dropping my bill by going this route.