Well with fountain pens, the sky really is the limit.  The most expensive pen I've seen sold at auction was around $30,000 - thankfully you don't have to pay such prices normally!  A serviceable pen can cost less than $10.00 - it will be simply made from cheap materials at that price, but it'll do the job and it won't break your heart if you lose or break it.  Personally I would aim to spend a little more and have a "nicer" pen - and try not to lose it!

Quality pens start around $30.00 and by the time you reach $50.00 you should be looking at an attractive, tough and well made item which will give many years of good service.

That sort of price will also buy you one of a great range of quality vintage pens.  A 50 year old pen will have the advantage of a solid gold nib and the fact that it was at least partly hand made will be obvious in its design and feel.  If the pen has been recently serviced and is in good condition it will be perfectly able to cope with regular use.  Models such as the famous Parker "51" or Sheaffer "Snorkel" remain popular simply because they are such good pens, a pleasure to use.