As to your supplier's pricing (Can. $)
Pricing is as follows for the items (or comparable) you've listed.
Aquasurge 2000 $287.99 retail, $269.99 wholesale
Tetra Bioactive Pond Filter 1500- UV retail $399.99, $303.99 wholesale
45mil DPDM liner (per sq.ft.) $0.99 retail, $0.79 wholesale (so a 15x15 liner, which is too small for your application, is priced right for retail)
Matala EzBio single-stage pre-filter $49.98 retail, $39.99 wholesale
I'm not a dealer, however purchase a lot of equipment from suppliers. I would suggest that you figure out what it is that you want, and then look on the 'net for a Canadian supplier and compare pricing. It doesn't really matter which province you order from, HST applies to all and over a certain dollar value the shipping is usually included. At first glance, it would appear that you're paying about $120 more (10-12%) than you should for the items listed, even at retail.
Now, given that you now have the information regarding retail/wholesale pricing on the above items, imagine what the margins are for these items.