WHMCS offers a 30 day money back guarantee, so it if you cancel in the first month it is like the first month for free. To get the license any cheaper than what they charge you would have to get it bundled with a hosting service, like a vps, a lot of hosting companies will sell whmcs at a cheaper price when added on to a service.
You might want to consider Blesta, they offer a 30 day free trial and then it is just $12 or so per month (I've seen distributors sell them as low as $7.80 per month also).
If you really want WHMCS though, you'll either have to pay the rates on their site or opt for a reseller / VPS / Dedicated server provider that offers them at a free or discounted rate. You'd be limited to hosting the WHMCS on their network though.
Also if you are looking for a great one, which provide reseller with free whmcs, and free comodo ssl and first month at $1 with coupon code: DOLLAR2016 after that around $18/month AFF link: client.innohosting.com/aff.php?aff=521 Just click on reseller plan.
Some reseller may provide WHMCS for free or at a discounted rate, however this will likely change over the coming months or years as whmcs enforce the new licensing fees into the few remaining resellers.
Although, some more expensive providers may continue to offer whmcs as a low cost addon for around Â£10-15 Per Month, despite been charged full rate for the license. This is because the more expensive web hosts are able to absorb the cost of whmcs licenses, through higher priced services and expecting a low amount of people to buy them, so the web host still turns a profit of the actual hosting they sold you.
I don't think it is possible to buy an "Extremely Cheap" WHMCS license. I'm pretty sure you have to buy it from WHMCS directly. As another user stated, you have a 30 day money back guarantee if you don't like it.
Also, as NameHero stated, you can get the license with a reseller account for free usually. This is probably what you're looking for.