Limited, but not fixed multisignature transactions, is that possible? Is it possible to have a multisignature transaction where N signatures are required and their inputs are limited in value, but not fixed?

The use case would be for a pooled acquisition, where the total amount to be spent is just roughly estimated, but not know. Every participant wants to just specify a maximum value to be used (probably the same for everybody).

The exceeding money, if any can go to a shared account or be returned to each of the "bakers".
Restrictions on the btc value of a transaction cannot be enforced via a transaction script. The btc value is not a property accessible from a script.

I think it would be possible though to create a protocol on top of bitcoin which does have such properties.