All in all, there are two problems with veteran contract extensions:
1. The salary in the first year of extension is limited to 107.5% of the last year of the current contract (that's about $13.9 million for Rondo)
2. Extension could be for up to 4 years total, including current years. Therefore, we can only extend Rondo for 2 years, given that 2 seasons need to end before his contract expires.
Therefore, the max possible deal on the table is technically 2 years, ~$29 million. I feel like the years are a bigger problem than the money, but in any case there's no need to wiggle around -- Rondo doesn't want to sign that, and he probably shouldn't.
edit: I see that this has already been discussed in more detail in the other thread.