You get what you pay for. That being said I would say you don't need an extremely expensive one if you are just looking for one to get the job done well. somewhere in the middle range would probably be best if you are beginning and only doing a small amount of grafts. From my experience most of the highly expensive ones are nicer but unneeded as the price's seem to usually reflects the knife having specialty detailed features, such as being engraved, having a hand carved wooden handle or other extras.
I also recommend getting a left handed grafting knife if you are right handed and vise versa if you are a lefty. Might sound a little confusing but I find this works best due to the fact you make the cutting swipe tword's you rather than away from you as you would when cutting just about anything else. And the flat side of the blade should be the cutting edge faced down when making contact with the scion. Some people even bypass using Grafting knives and just use disposable razors discarding it after every few cuts. Never myself used it though so i cant give my actual two cents on what is the best method but I thought it would be worth mentioning. Basically I would say it is just important to not get a cheap on as you want to make sure it keeps its edge well since in order to be the most effective you want it to be razor sharp. I always sharpen it to the point of being able to cut the hair in half on my arm by lightly rubbing it on one.
Don't know it this was helpful or too much of what you already knew. Sorry I'm rambling again