I recommend a review of any potential reader‘s or company’s training and professional experience followed by a request for current references. Free or cheap coverage is worth what you pay for it. There is no need to suffer insults. A good coverage is more an audit than a critique.
It takes approximately one hour for a qualified reader to digest a standard format 110 page script and another hour and a half to go back and mark up the script then provide a separate quality write up. A disinterested third party is your best bet to be dispassionate in this process. Referrals from professional writing groups (as opposed to groups of reading buddies and relatives) or professional “meet-up” groups, like The Table, are a good start. University writing programs may also have references. Requesting comments on your first ten pages, with the cost applicable to the total coverage cost, is not unreasonable.
A competent reader can tell within the first page if a writer has the chops. Purchasing this service is a valuable investment before you lay out anywhere from $120 – $1,500 for minimal to detailed production notes. Production notes, however, are a waste of time if your script fails within the first ten pages. Trust me there are those who will be glad to take your money at all levels.
Disclaimer – I provide this service and have for over 3,500 scripts but strongly suggest you request a sample redacted ten page coverage before you send any money to anyone.