Guitar Pro is good to start with; when I got it (back in Version 5 days) I believe I paid $49.99. A bargain considering how huge a leap it was from writing by hand, and you can download tabs for songs from places like 911tabs.com that you can open with GP like a score, so you can play along with it, read the notation and change the tempo if you wish.
If you're doing a lot of "idea stamping," i.e. jotting down musical ideas you come up with on bass, it's great for that, and you can also compose pieces by adding other MIDI instruments and writing parts for them. If you plan on doing serious composing, and have the capital, I recommend Sibelius. It's immensely powerful and intuitive to use (easier than Finale) although it has a hefty price point. Not quite as simplistic as GP, but it's much better for full-scale compositions with large ensembles. I don't even want to think about how my 12-tone piece for Pierrot Ensemble would have turned out by working in GP...
