Excluding Scottish MPs from voting on budgetary changes that affect Scotland would be outrageous. However, that is clearly not what is proposed.
The proposal is that Scottish MPs do not vote on matters that do not affect Scotland. It follows that if Scotland was affected, its MPs would get a vote.
The budget is set for the UK as a whole, and Scotland is allocated a proportional block of funding. It is up to the Scottish government (acting within its powers under the Scotland Act) how that funding is spent. Therefore Scottish MPs would get to vote on the budget (and budgetary changes).
It's no different from devolution, just an awful lot cheaper.
Coffeethrowtrampbitch
The Scotland Act determines what lawmaking powers are delegated from Parliament at Westminster to the Scottish parliament. It follows that all Westminster MPs get a vote on amendments to it.
As for the power to hold independence referenda? Really? How often? Every week? I don't believe the Tories made any such promise, but I can imagine lots of people in the indy blogosphere claiming a promise was made on zero evidence.