I would take into account a few things, like:
How long you guesstimate it will take to earn that $1500 back using that twitter account, and whether you could do the same with a similar twitter account name
If it's a cool keyword name account you are considering buying, perhaps it might be almost as good to just choose a longer twitter username with an underscore or dash and some other word or perhaps numbers
e.g. keyword_word2
Does it give you all that much more credibility with your target market if you have the $1500 keyword username?
There is the intangible value of "brandability" to consider, and how much value you place on that over during the course of building this business.
How responsive and savvy is your target demographic with regards twitter?