I did something similar with a girl I was conversation partner to last semester. The thread where I had everyone's advice is here
if you want to take a look.
I can post a scan of the breakdown we did in a notebook, too, but that will have to wait until I get home and dig it up, haha.
Edit: From the thread I posted right there, the final breakdown of what we wound up getting was as such:
Coming up to about $31
$5 Water treatment
$3 Fish (Female veiltail betta)
$1 Water-change bucket
Then I gave her:
A measure of my betta food (Otherwise $4ish)
A length of airline hose for water changes (Otherwise $5ish)
A thermometer ($4 Petco glass one with suction cup)
Some plants (Gifted to us by another forum member)
A plastic rock and plant decoration (Which had been un-used since I started with the real stuff)