Then you've got a techy sort of guy with a few measly thousand C-Bills saved up who buys a heavily used Scorpion tank....and starts a Corporate Security mercenary company, just him, a "gunner" who's never fired a gun, and some ex-military guys on foot who can at least shoot a rifle. They get paid enough for guarding an industrial appliance factory to cover salaries plus a few thousand more over the course of a year, and the tank crew gets enough time at the controls to actually learn how to aim the guns in the right direction and steer the tank around basic obstacles. Next contract, some local robber gang shows up, with a bunch of converted trucks (a 'Mech-grade machinegun on one, and an actual SRM/2 rack on a second, and guys riding on the back of yet another truck) to steal equipment from the factory, and the Scorpion puts a few big autocannon holes in one of the trucks. The rest run away. Now you've got a reputation, some damaged weapons and metal scraps for salvage, and the contract money to hire on another "owner/operator" tank crew that couldn't make it financially on their own, or who didn't get paid by the last merc group they worked for....and so on. Now you're in the 'Merc business, and you don't even own a 'Mech yet. Another good contract or two and you've got a Lance of armor plus infantry support.
Or how about the ex-mil guy who pilots 'Mechs in the arena, manages to win a couple of fights, and suddenly has enough cash to buy his own 'Mech. Suddenly, risking everything in the arena doesn't look so good compared to parking the 'Mech next to a factory to scare off would-be thieves. With a passably decent contract, one doesn't need to actually fight to make sufficient money to gradually expand, by adding "support" like an armored vehicle or some infantry after the first year or two.