The only charges on paypal are transaction fees that are deducted from the amount sent. For example, if person A sends person B $100 on paypal, then person B isn't going to actually get $100, but rather more like $98.50 or something like that (I don't remember what the exact percentages are, but it's not really all that much). Person A will still only be out $100, though, so they won't even notice a difference.
There are zero monthly fees for using paypal, or any other charges. It's just the transaction fee, which only the recipient of the money will notice.