uc/os-iii starts from 6000$
I am starting some free tutorials about ucos-iii to build on ucos-ii @ Embedded Tips from Amr Ali if you are interested.
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Amr
SALVO RTOS (Pumpkin: Salvo, the RTOS that runs in tiny places) is around 1000$ - it can fit from extremely small uP (like PIC12C508) to top range uP. Available for many differnt uP. Supreme functionality and small size, excellent documentation and rock solid behaviour. Once you try SALVO you'll not want to use other RTOS.