Outside mastering will cost to get it done right. So I guess the main question here would be.... What do you plan to use the mastered song for?
If it's personal use, or to post online or some other thing that is not going to pay for itself..... I don't think I would pay for mastering.
If it's guaranteed to be signed and cut, or going on a commercial release then yes.... it's worth the money if you can not do it as good as the mastering house.
That would be my criteria for making that decision.