The explanation why you see that kind of price for the boost is that playing Rocket League is a sign of longevity. Only those who helped test Rocket League in the PC only edition of the game had the boost available. So you could get the reward while the game wasn’t quite over, and players were testing until the game was a public beta. It shows that when it comes to the game, you’re basically an OG. It’s also the reason why the cosmetic never came back to the market, and why for a long time it kept the same price.
You should try your luck in the item trading area if you want to get your hands on an Alpha Boost, but there are slim chances that you will get it. The Rocket League’s rarest items have begun to raise the price scale, and it may get higher as time goes on.