"...most warrants trade at about 20% of the share price until near the end of their life..."

Not sure if would agree with that. When far away from expiry, then 20% looks like a good rule of thumb. As they get closer, it depends on a number of factors.

The following site has prices for Canadian warrants and their underlying shares. You can sort by all kinds of things, including expiry dates. Just hit the various tabs.

resource warrants: http://canadianwarrants.com/values/c...

fund/trust warrants: http://canadianwarrants.com/values/c...

I find the site very useful.