There is no right or wrong answer here. You should ultimately look to spend as much as you feel comfortable spending. There is, however, an unofficial guide that says you should aim to spend 6-15% of the cost of a brand new guitar.  It's also important to bear in mind that buying a solid case for your instrument is an investment in itself. If you have a Telecaster that is worth a sizable amount you definitely don't want it getting damaged unnecessarily.