boredom whilst by myself.
I always looks forwards to jamming with my band, always hoping the drummer remembers his recording device so I have something to work on during the downtime.
Otherwise I play normal guitar, and I don't have the want or money to play that seriously, so little progress is made by myself (unless of course, the beforementioned circumstances).
oh yes and as just mentioned, not only the cost of strings, but the difference in sound quality of new vs old strings on a bass. My guitarist buys 5 packs of strings a go and that does him for a year. I buy one pack and make them last half a year if I can (boiling etc). My guitar strings are a year old and compared to some fresh strings on a guitar they sound fine. Bass strings after 2 months sound super flat and dead after 2 months. The difference is huge.
Of course, theres also the fact that all these things I go through to get the perfect tone (like, the difference in punchiness of new vs old strings is huge, and imo makes a huge diff on some of the songs we play), I don't think ANYONE outside of bass players who have been playing a few years notices or gives a crap about such a thing. The song would sound completley horrible with a super smooth tone, but no one cares enough to notice.
Thats where your money goes. To hopefully entertain people who wouldn't notice the diff between a 15 watt practice amp (besides volume) and a 1000 supermegawatt amp.
Such is life.