the sales people will help you and will explain everything to you.
the prices vary greatly by type of frame you pick, type of matting you pick (if any), whether or not you're doing archival framing, the type of glass used, and whether or not the framers at the shop are certified custom framers and whether or not the shop participates in a custom framing guild or professional organization for custom framers. your price range could be as cheap as $20 and as expensive as several hundred dollars.
standard sizes and ready made frames cost the least. 9x12 is a standard size. If you want a mat, then 11x16 and 16x20 are the next 2 standard sizes up from 9x12.
there is regular glass, non glare glass, plexiglas, and non glare plexiglass. the non glare glass is usually treated for blocking UV as is the plexiglass. regular glass is the cheapest. non glare glass is for something that will have a light shining on it or be in a sunny location. plexiglass is lighter than glass but it scratches very easily. plexiglass is usually used in large pieces where the weight of the glass would tear the frame apart. plexiglass is never used over pastel chalk pieces because the static electricity from the plastic will pull the chalk off of the paper.
depending on how busy the store is, how much stock they keep on hand and whether or not they build their frames there or send out for them, they could complete your piece in a few hours to several weeks. the closer you get to the holiday the busier the store should be and the longer you will have to wait for your piece to be complete. (if the store isn't busy for the holiday season then they aren't very good at what they do.)
worked in frame shops through college. ran frame department for several galleries and museums.