$1.99 is a VERY good price for Muscle Machines! I'd snag a bunch for customs.
I remember when Dollar Tree had them, but I think it was 2007. Maisto had taken over, and they started out ok with the first and second series, but by series 3, they were getting hard to find. (just try finding a Vista Cruiser chase car from series 3) Series 4 and 5 were the ones that landed in Dollar Trees, with the East Coast getting series 5 and other parts of the country getting the 4s. There were TONS of them on the pegs! I remember putting together at least 6 sets of series 5, for other collectors out west, including chase Chevelles.
I ended up finding the series 4 cars on a trip out to Pennsylvania.