Personally if it is making you money on a monthly basis then keep it. It seems like atm it is a guaranteed amount of money for no work. If you sold it and then you just blew your money you will be upset that you were not getting the $300 coming in each month.
With your update posts it seems like the site is getting better which is great, congrats.
A tip to note is, the more your site makes, the more it is worth. If you can keep the $600 or so up that will mean you have doubled your sites value.