The Pentax FA50mm f1.4 and Nokton 58mm f1.4 would be two lenses to consider seriously for low light. Their price with K-mount (as new lens) is about the same at Adorama.com (~US$350), although slightly above the upper bracket of your price range. Second hand prices will be lower.
The FA50mm is AF. The Nokton is MF. Their reviews are at photozone.de:
+ "The Pentax SMC-FA 50mm f/1.4 is an oldie but a goodie. [...] Regarding its very low price tag it remains a highly recommended lens. "
Pentax SMC-FA 50mm f/1.4 - Review / Lab Test Report
+ "The [Nokton] build quality of the lens is superb. [...] The lens is absolutely beautifully crafted and a joy to use despite the lack of AF."
Voigtlander Nokton 58mm f/1.4 SL II - Review / Test Report
I needed a low-light lens of my K-7. Between these two lenses, I chose the Nokton 58mm. I am delighted by the superb Image Quality, and excellent low light performances.
Hope that the comment will help...