All-grain brewing for *small batches* isn't an expensive investment. A 5-gallon cooler, a plumbing supply line, and some spare parts, and you're in business. You will make it up in the savings over extract very quickly, and you have the fun (if you think it's fun) of mashing, plus the ability to make any recipe.
What I wish I had a do-over on was thinking that I really needed the propane burner and the 8-gallon pot. I've used them to make good beer in the 4-gallon range, but for a variety of reasons I've found myself moving back to the kitchen to make 3-gallon all-grain batches (thereabouts, a little more, a little less). Dealing with more than 3 gallons is physically uncomfortable for me, and in the kitchen I don't have to deal with weather variables. I went back to extract for a couple of batches and it really didn't save me much time--it took longer, but it didn't add that much labor effort. The last all-grain batch I brewed, I mashed the night before, went to sleep, and had the wort boiling before 7 a.m.
Instead of that burner and pot, I could have invested the same money (and space!) in a small fridge and the parts for temp control. I live in a part of the country (San Francisco) where most of the year you can ferment pretty decent ale by putting your carboy in a garage on a cement floor, but I'd like to dial it in more precisely than that. So that's my next stop.
The books have been a great investment. I did a lot of business travel for a couple of years and was constantly pulling a brewing book out on the airplane or in the hotel room to re-read a chapter and puzzle my way through a concept.