Is the hair coarse? What part of your arms? Is it just one spot with hair and no hair on the other arm in the same place? If so, do you know why?
First, the answer is that electrolysis is probably going to be cheaper.
$120 per treatment on such a small area for 50 hairs is really too expensive in my opinion. Also, you mention anothing about the type of laser being used, which is very important. There is a place in Allentown, Pennsylvania using GentleLASE run by someone who used to be a consumer on these forums. Their site is http://www.gethairless.com/
Either way, for 50 hairs I wouldn’t bother with laser, especially if the hair is not dense and coarse, which it doesn’t seem to be for an area of this size.
You have to understand hair growth cycles. You’ll need multiple treatments with electrolysis and will likely need to come in more often for electrolysis treatments as you need to treat the hair as soon as it comes out. On average, an electrologist removes 5-10 hairs per minute and charges $65 or so per hour. So Your area can be treated for around $20 per treatment. Even if you have to go in more often than with laser, it will likely still be cheaper than $600 overall. And most importantly, it will be effective on finer hair.