300 bucks for a bosun's chair? I guess if you want to you could spend that much...
But for the fiscally challenged, a bit of stout plank, perhaps 8 to 10 inches wide and long enough for your bum to sit comfortably upon it, some 1/2 inch or so line to make little "bridles" at each end that meet in a hefty steel
ring at about chest height, all spliced neatly together and you have what most folks used for years to go aloft.
You can pimp it up with a padded seat and perhaps a strap across the back to lean against, even a crotch strap to keep you from sliding out, but for short trips and simple jobs the basic "swing seat" will get the job done.
For the more affluent, a trip to a climbing shop will show you some fairly inexpensive climbing harnesses that are likely safer (in that they are professionally built and sold, with liability issues addressed) and not terribly dear. Many riggers use them (maybe not the bottom of the line type that are cheap); I find them uncomfortable for long times aloft.
But, the point is you can get up the mast for far less than 300 bucks!