The reliable way to gauge spot cash value is to compare your item to sold comparables on eBay
. Many sellers place dreamer prices on their wares, so I prefer to see what a "willing buyer pays a willing seller".
I found several blocks of four selling in the $6-15 CDN range, but you can check this for yourself. The cancel can make some difference to value - a nice CDS or tidy oval Registered cancel is a plus. The roller cancel is common.
A buyer of used blocks is interested in the usage, thus I'd reaffirm they'd pay no premium for gum. If found on a large parcel piece or bank tag etc., its market is a postal history collector.