Mostly it's what they're calling a lead, then the channel, then the market, in that order.
PPC means pay per click. You're charged when someone clicks the ad, not when they contact you. On seller-intent search terms, the ones where a homeowner types something like "sell my house fast" plus a city, the click can run anywhere from about $10 in a thin rural market to $60 or more in a metro with a dozen funded buyers bidding. A landing page that turns 8% of clicks into a form fill or a call at $40 a click puts you around $500 for one raw contact. At 15% conversion and $15 clicks you're near $100. Same discipline, same math, very different answers.
Then there's what counts. A raw lead is anyone who submitted a form. A qualified lead usually means someone who owns the property, wants to sell in some timeframe, and answered the phone. Agencies that quote $30 are almost always quoting raw form fills, often from Facebook lead forms where a person taps a prefilled button without much intent. Agencies quoting $300 are frequently quoting a contact who actually spoke to a human.
The number that decides whether any of this works is cost per contract, not cost per lead. If 25 raw leads produce one signed purchase contract, a $100 lead is a $2,500 acquisition cost. Before you sign anything, get the lead definition written into the agreement, because a vendor paid per lead and left to define the word will define it generously.