It is t always about budget. Sometimes it’s just knowing people in the industry. A good example is Red Vs Blue had a pretty low budget all things considered, even when it got more popular, but RT managed to get Elijah Wood to voice a character for a few seasons since he knew a few people at the studio and liked the show. That was before RT got bought by Fullscreen/Warner and was given bigger budgets mind you.
Same thing when they got Yuri Lowenthal (known for voice-acting in big stuff like Ben 10, Spiderman, Saints Row, etc.) to voice a character in RWBY back before that show got popular (early in season two after a former cast member left, and Yuri kept voicing the character until the show went on indefinite hiatus)