Bad coaching is at best a waste of money. Good coaching is priceless. What really screws things up is that a particular coach may be horrible for one rider but the best thing since sliced bread for the next guy. The problem is that each coach has a formula and that formula is not a universal solution.
Self-coaching also has its own pitfalls. You can read all of the books that you want, but there is no single training method that works for everyone (see above). We are all individuals and we all respond in our own ways to the various training stresses. This takes years of making mistakes to eventually figure out what works for you.
If you are an analytical mind and will read a lot about different training strategies, then spending the $'s on a PT is probably money well spent.
If you get a coach the first thing that he/she will want you to get is a PT since it is very unlikely that they will ride with you to get a gauge on your effort levels and fitness.
Do the math.
As for me I have been riding for so long that 10-speed has changed its meaning. I don't need a coach or a PT to tell me that I suck and always will. I already know this from years of painful experience.