This is the wrong way to look at it.
Instead, ask yourself: would you exchange 7% of your company to join the YC community and be able to leverage their resources forever?
The answer should be a resounding yes if you think your company will be > 7.5% more valuable if you join YC [1]. Which it should [2]. The $125K is just the cherry on top and just one of many perks of joining YC (albeit a useful one for companies that have no funding/revenues so they can focus 100% on building their product instead of having to worry about paying for housing/food/servers/SaaS).
The vast majority of us who have gone through YC would've done it even if it wasn't for the monetary investment.
[1] See PG's Equity Equation essay: http://paulgraham.com/equity.html
[2] You'll likely even make up for the 7% almost immediately because you'll likely raise your seed round at a significantly higher valuation (> 7.5% higher for sure) than if you hadn't gone through YC. But it's very likely that your company will intrinsically be worth significantly more than that too.