Having a go at mastering is easy but doing a good job is not! In a way it's a deceptively difficult job.
When paying for a pro mastering engineer, you are mainly paying for their experience and ears.
Mastering processing techniques tend to be more complicated than basic mixing - multiband compression, mid-side, dynamic eq, soft clipping and limiting etc...
Plugins can be helpful but be wary of any magic bullets. Every mix is different. IMO most difficult part of mastering is knowing when NOT to use the tools available to you!
Not meaning to put anyone off, but there's definitely more to it than using a few plugins on presets. Then again, depends on what you're after