Need novice motorcycle buying advice.?

If you want to go with the best bang for your buck then you could easily find a nice 600cc or even a liter bike for around $4k. Small bikes like a 250 and cr125 depreciate very quickly because people usually beat them up and they are mostly for beginners, I suggest searching for 03-06 600cc depending on your tastes (naked vs. full fairing) you'll be able to have many choices. Late 90s to early 2000 R1s go for about 4k and fuel injected is what you are going to get best power and fewest troubles from. Don't worry about the gas mileage. Most sportbikes these days have about a 5 gallon take and most get close to 60 miles per gallon.