True, it is always hard to get the taxpayer fund transit. However, the dependence between the cost of a new line and the level of tax push-backs is very non-linear; sometimes, getting a cheaper line funded is as hard or harder than getting a more expensive version funded.
I don't have a strong opinion on the Brampton transit options, as either solution won't affect me. It just seems to be that, should the tunnel proponents prevail, transit advocates should't go ballistic. Even if the tunnel solution is not optiomal on the cost-benefit scale, it would still increase the transit coverage and improve the connectivity, versus being locked in the debates and having the line terminated at Steeles.