They can be helpful, eventually.
If you're in the beginning stages of the PB and are still ironing out the basic plot of your story then it's probably not worth it. You might have to make major changes which basically change the whole story, be asked to start over, or they'd critique it but then say it's not likely to sell. Although learning experiences, one has spent the money.
Like any critique - it all depends on what is received back - and you can't really tell how useful it would be till after the fact. A quality critique is a quality critique, free or paid... Also, it's still someone's opinion so there is that subjective element as well.
However, I would expect that they know more about the market than the average picture book writer... but again, anyone could figure this out too but it takes time and effort to see what's selling and what not. In a way, you're "spending" your time and effort rather than spending money.
After a couple of rounds of free friend workshops, then it might be something to look into.