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!