Many professionals will say that (in usual cases), the higher the price, the better. Andy Martin of Martin's Magic (and also a world famous magic collector) has said that to get something great, it must be at least $100.00. Some other pros say that as well, that the more expensive, the better. Sometimes, this may be true, especially for teens and adults. But for kids, there are still some expensive effects that work really great and may exceed the less expensive tricks in the entertaining impact they have. In general, an expensive flying scarf (silk) that flies through the whole stage and even some/all of the audience will have a higher impact than a cheaper scarf (silk) that can only fly a foot or two away from the performer. That is, providing the presentation is equal between the two performances, then the more expensive scarf will have the higher entertainment, I'm pretty sure.
Of course, this is not always the case. For real little kids, simple and cheap things which would be considered kid friendly are often the best. If it's so expensive, it can look too complicated and not kid friendly. Even for older kids, teens, and adults, things under $100.00 and even under $50.00 are often just as great as those tricks that are in the hundreds and thousands of dollars range. Tricks like card tricks (gimmicked in some way) are perfect examples of tricks under $50 that can deliver a high impact like those more expensive effects. But, as with my flying scarf (silk) example from above, if you have the money and a certain effect you're interested in has a cheap version and an expensive version and if that expensive version would have a higher entertainment impact on the audience, then by all means, go for that and give the maximum entertainment you can.
Really, look at yourself and your personality you would bring to the stage (show), and from there, go for the tricks that you would be able to work with (and that you like so as to have a personal interest in them) and price probably won't usually, if ever, be an issue and you can make the best of everything to give a very entertaining and successful show (some expensive tricks could be better as written above but are not by any means a necessity for a successful and entertaining show).