The way the Canon Refurb store works is any lens, body or flash that appears there is sold at 20% off Canon list price (the price you see when you look it up on the Canon site). Normally, this is a bit under what the big camera stores charge. The caveats are: only a 90 day warranty, you do pay sales tax for most states, you will usually pay shipping costs, and, of course, there may be slight cosmetic imperfections on the gear. But, you will still see savings. I bought a 100-400 last year, and even after tax and shipping, it cost me about $150 (about 10%) less than the big camera stores charge just for the lens. You'd have to work it out for yourself if that's enough discount.
The current sale is 15% off the refurb price, so, effectively a 32% total discount from list price. And the tax you pay (if you do) is based on the 32% discounted price.