At what dollar threshold do you require an executed contract for a service procurement vs a purchase order? Also, at what dollar threshold do you require legal review of the contract?

As far as legal review is concerned, we require review above $100K.   Below $100K we can use legal endorsed templated agreements.  With regards to purchase orders, we can use those for one-off purchases.   If it is repeat spend, we required order forms or agreements.   The spend threshold is commodity and case dependent.   

In my experience, we used POs for spend up to the five-digit level, moved to formal contracts once we hit the high five-figure range, and looped in Legal at six figures (or sooner for new vendors or IP/data-heavy agreements).