Roberts Oxygen on Triad Drive (just off sandy ridge road exit) does the 5 pound tank exchange for $8 ($8.62 after tax). I just swapped tanks a couple of weeks ago, so the pricing is valid. They also exchange 10 pounders, but I do not know the pricing on them.
This is the guy I asked to let you know. :-)
$8 is a much better deal.
55 gal is an awful tank for aquascaping, but they are often cheaper because they are a popular tank and more are sold.