@Account Closed is exactly correct. My wife and I are both licensed agents here in SA and $100-$150 is totally normal for an option period. We would never advise a client to put up $1500 for that, no way. If your property is in good condition and you've completed the Seller's Disclosure completely and truthfully, the option period shouldn't worry you, especially if you're in a multiple offer situation. Simply select the offer that appeals most to you and allow the buyer to perform their due diligence. Then work with them to address any issues that came up during the inspection. If you have multiple offers, you can also put one of the others in a back-up position (if the buyer agrees) just in case the first offer doesn't work out. It also sounds as though you may be trying to FSBO this property. Situations such as this is why a good, experienced agent is often worth every penny. Selling a house isn't as easy as it sounds.
Also, as Greg mentioned, $300K really isn't high-end. That's typically a standard home in an everyday middle-class neighborhood here. For "high-end", you'll be looking at double that amount as an entry point.
Best of luck