The gallon will be extremely thin and just barely enough, if it is actually enough.
We use Linex and Superior on our company trucks, with Superior being cheaper. Rhino will turn grey and chalky much sooner than the other two.
If you are gonna DIY, use Zolatone. The stuff in my truck is 20 years old and looks better than any bed liner I have seen. If you manage to scratch it, touch up or reapplication is very easy.
If you buy a $20,000+ new truck, don't get cheap on the bed coating. You saved enough money by not getting a V-8 and power windows to pay for the bed liner.
Lastly, at least use the bed and scratch it up prior to the coating. That will give you time to save some extra scratch for the right stuff.
$450 is too high for a short bed. $500 is highway robbery. Shop around and work the dealers against each other. All too often they get used to customers and dealers just paying full price. A new truck is very little labor and they can do several a day.
Possibly, your dealer may have a connection and can set something up for you.