There are different answers to the question. The key is to remain realistic with the answer. Cheapest that does the job, right?
For a student or a would-be luthier, I'd recommend an SX or Affinity, no problem. Used if available to check out first.
For a semipro who's gigging locally I'd recommend Squier, new or used. I know a PHX guy who sounds great on his modded Douglas, too. That's one of his "around town" basses.
For a touring pro, I'd probably recommend a used Carvin as the cheapest fiver.
For all the praise the cheap basses get, remarkably few top pros have them on tour. The road is not the bedroom, the basement, or the garage. They cannot afford to bring johnk_10 on tour with them. The 30 hr you spend making your SX sound perfect cannot be replicated nightly before each show when you bring 4 basses. THAT is the price difference right there.
Seriously, tone isn't the only thing; it's one thing. Same with looks. Reliability and repeatability are major considerations when you use the instrument all the time. Stability under environmental changes, availability of parts, ease of maintenance, quality of materials and preparation (bake times and profiles on wood!), and resistance to abuse are all important. Take your SX (or your Realistic JimmyM

) to 40,000 ft. and -40F and soak it there for a few hours; then return it to STP. Do that with an MIA Fender or Carvin. Notice any difference?
There's more than one correct answer to the question, no offense intended.