What do stitch guides cost? The answer is, it depends! Stitch guides can be free. Shops will often do a simple guide for you if you buy a canvas or the threads they recommend for it from them. Some designers have free stitch guides on their websites for some of their designs. Don't expect a really detailed guide, but many of them are well done.
Stitch guides can run $10-50 for guides that have been done for a designer by a professional stitch guide writer. The price varies because canvases are all sizes, shapes and levels of difficulty. Often stitch guides can be ordered from the designer when your favorite shop orders you the canvas they go with. Many times you can ask the stitch guide writer to sell you the guide directly. Most (but not all) are happy to do so, but a few will not due to the way they structure how they work with a designer. It never hurts to ask, though. If the guide is only available from the designer they will tell you so. And you can always ask for a price and some idea of what the guide is like, whether it is suitable for a beginner, etc. Most folks are happy to give you some idea of the cost and what you are getting.
Stitch guides written for an embellishment class can often be purchased after the class from the teacher. Tony Minieri, Amy Bunger and Robin King work this way, for example. (None of them do custom guides any more except for folks who sign up for an embellishment class.) So if you have seen an embellishment class piece you are in love with, check with the teacher to see if the guide is available from them. Usually you will have to source the canvas and threads yourself unless the teacher has their own shop like Amy Bunger does (Amy's Golden Strand in Memphis, TN) or you are talking to the shop that hosted the retreat or embellishment class.
In the case of a shop, you often have to buy the canvas and/or the threads as well as a package deal. A shop will tell you how they prefer to work if you ask. These embellishment class guides are usually fairly expensive. Folks pick large, complicated and difficult designs to take to embellishment classes so you will pay top dollar, usually anywhere between $85 and $300 for such a guide. They take a long time to write and normally the stitch guide writer will only sell 1-2 of them instead of the more usual 5-20 copies of a guide written for a designer.
Custom stitch guides are the high end of the scale for stitch guides. They tend to fall in the same price range as embellishment class guides, i.e., $85-300. I think the most I've ever seen charged was $350 for a really large and complicated piece. The guide was nearly 50 pages long. For custom guides, some stitch guide writers charge by the hour, others charge by the page. By the way, if you are ordering a custom stitch guide, don't be surprised if the stitch guide writer sells it to someone else later. That's one way a stitch guide writer recovers payment for their time. I have never ever written a stitch guide that paid me even minimum wage given the number of hours it took me. You can of course offer extra payment for the guide writer to not sell it to anyone else. I used to offer a discount for custom guides I could sell to other folks when I wrote custom guides but I don't think this is usual practice.
If you have questions about a guide, ask. The stitch guide writer will tell you the approximate cost, what level of stitcher the guide is geared for, and even how long and elaborate the guide you will receive will be. Sometimes the stitch guide writer can tell you if you will use things like needle felting, needle lace, beading or other special techniques. If you don't have a large stitching library, ask if stitch diagrams are included in the guide. Usually a guide that includes diagrams costs more as graphic programs to create stitch diagrams are quite expensive and difficult to master.
Don't forget to check the website or blog of a stitch guide writer when you are looking for prices. Many list all the guides they currently have available with the price. So take a look at the Stitch Guide Writer Contact Info tab and follow the links if there is a URL.