Is it possible to create a "global wishlist"

I have several projects, and at least one item in those projects is usually something I have to buy. I’d like to link these items to a separate project that’s a wishlist so that I always know how to budget my money.

Is this possible?

Assuming you’re using the Pro version of OmniFocus, the easiest way to do this would be with a custom perspective. You can build a custom perspective that searches for something specific, so you could either just look for tasks that have the word “Buy” in them (this is what I do for my shopping lists), or use some kind of pseudo-tag like #tobuy or #wishlist in the task name and build your search to look for that.

If you’re not using the Pro version, about the only other way I can see to easily do this would be to create a separate context for your “Wishlist” items and assign those to that context.