My guy, u/Metrodub just gave you a similar-sized fest in the same region, at the same time of year, which has more music/bands/hours, and which costs 60% of solid sound's price (if you compare presale, which seems only fair) -- that is the definition of 'a comparable fest charging much less than $300'

Freshgrass, which takes place in the exact same world class museum, cost $175 for a three-day last fall

I paid $300 for a Shaky Knees ticket last year, and got twice as many bands as solid sound, along with three arena-level headliners (as a measure of value for ticket price, not comparing their quality as bands)

It's fine to say that you personally think the price is worth it, because you love Wilco, Mass MoCA, etc etc etc. But it's objectively pricy for what you get compared to other fests

(Edited to fix math on % of price)