For others considering Island Birds, the cost has a lot to do with the size of the airplane required for your charter. For comparison (although this pricing is 1 year old). We were quoted $1575 Beef Island to SJU and $895 Beef Island to St. Thomas for 3 people flying in one of their 5 passenger airplanes. They offer a discount that seems to hover around 10% for repeat customers ($1400 to SJU and $820 to St. Thomas).
It is pricey anyway your look at it but using them can allow you to make connections that you otherwise could not and they make the whole travel process easy by walking you through customs on both sides. They have always been very reliable for us and have provided great service all the way around. I am about to check on pricing for our next trip to paradise!