Getting bacteria or animal cells to express a custom strand of DNA like a fluorescent protein is basically a first year lab in undergraduate molecular biology. There are even relatively cheap consumer kits made by the diy bio community [1]. DNA synthesis is at most a dollar per basepair, although you'll have trouble finding someone who can accurately produce something approaching or exceeding a kilobasepair.

[1] http://openwetware.org/wiki/DIYbio/FAQ/Kits