If your net income is about $150 per month, you can expect to receive about $2700 (projected income for next 18 months) if you sell your site on a platform like Flippa.
if the cost you have to pay monthly was bigger than revenue $150 you got, probably you have to sell it.
Your site is earning site, may be small earning, so what! Why should you sell it? Better upgrade it and keep continue, you will earn a lot, I wish.
It depends on your site. If it has quality backlinks, content, stable traffic flow it can cost $2500+. But I think you should work on the site, improve its metrics and income, only then sell it.