The closest alternative to Sonar is GPT-4.1 Nano — comparable intelligence (9.6 vs 9.5); for a cheaper option, GPT-5.6 Luna runs $0.500/M vs $1.00/M; and the best open-weight substitute is Kimi K3.
GPT-4.1 Nano is the closest alternative to Sonar, with an Intelligence Index of 9.6 (Sonar scores 9.5) at $0.100 per million input tokens.
Yes — GPT-5.6 Luna costs $0.500 per million input tokens versus $1.00 for Sonar, while staying within ~8 points of its Intelligence Index.
Kimi K3 is the strongest open-weight alternative to Sonar — downloadable from Hugging Face and self-hostable, scoring 57.1 on intelligence.