The short answer is no, you shouldn't. Or, if you do, don't expect to get your money's worth.
This may sound overly skeptical, but it's unlikely that the advice you receive from a script consultant will be worth the price. If you want to improve your screenplay as best as possible, there are far cheaper alternatives.
Paying upwards of $300 for someone's advice is fundamentally a little desperate, even if you have the money. It's true that using a script consultant will be a good learning experience. A least, for example, you'll learn how easy it is to be ripped off bypeople claiming to help you write better.
Consider for a moment the ever-growing industry that hopes to profit off would-be screenwriters. Even the best intentioned script consultants secretly laugh all the way to the bank as they provide generally obvious advice to eager novices.
Before you fork over a bunch of money to a script consultant, werecommended that you first:
Fix Your Script Yourself
Be your own best editor by putting the time in. Write a number of scripts (say, at least five), and edit each one as if someone were paying you $300 to do it.
Anecdote: A friend of mine worked for several years at Apple's in-store help desk. Most of his customers were middle-aged women who were educated and intelligent, albeit not terribly tech-savvy. After determining the nature of the problem, my friend would ask, "What would you do to fix this?" They would respond "I don't know," but he would push them to do the thing they thought most logical.
Nine times out of ten, they would end up fixing the problem themselves when forced to take some action. This is to say, they knew the answer all along, they were just too hesitant to take the corrective action because they didn't consider themselves to be "experts."
Take this to heart when editing your screenplay. Sure, an expert could help you fix your third act issue, but I'd be willing to bet that you already know the answer yourself. After all, you've already trained for this moment by watching countless films and probably even by reading Robert McKee's Story or other screenwriting books.
Cheaper, Better Alternatives to Script Consultants
If you need outside advice from somewhat non-partial people:
1. Join a screenwriting Meetup group. If you live in a big city, you can probably find one. If you can't find a Meetup group in your area, start one. From my experience, random people at screenwriter Meetups are more than happy to tear a script to pieces, even more brutally and expertly than high-priced consultants.
2. Sign up for a screenplay workshop or a class at a local college. Screenplay workshops often cost a decent amount of money, but, unlike consultants, they're worth the cost. Rather than being handed down notes from a single person, you'll receive feedback and support from a number of peers and also an instructor of some kind. These exist even outside LA. For example, in San Francisco, a quality screenwriting workshop is hosted at The Grotto.
3. Find low-cost script analysis services. By searching around online, you'll find a number of alternatives to high-priced consultants. For example, SellingYourScreenplay.com by screenwriter Ashley Scott Meyers offers a script analysis service starting at only $67 bucks per script (or less, if you sign up for having three scripts analyzed).
If you still decide you'd like advice from a full-priced industry consultant, proceed with caution. Start out by listening the the consultant's podcast (see: Best Screenwriting Podcasts). If the consultant has written a book, check that out, too. If you decide you that don't share similar aesthetic values with the consultant, then you probably won't fully appreciate--or grow from--their advice.