hi dunia.
the price of mechs is "relativly" simply. most mechs you can roughly count by tonnage in million c-bills +- 10/20 percent from the factory. xl engines double that price, xxl engines triple it. electronic devices such as a c3-system, targetting, ecm etc range into millions, too. afaik a c3 master counts as 5 million. thats roughly the price. many are listed over at sarna. and of course there are certain books to do the numberscrunching yourself ;) ;) have fun counting hehehe
but for your typical merc unit the price is not the most important factor. in the various editions of the rpg, characters can accumulate quite a lot of money and vehicle traits to own mechs. the problem of acquiring the mechs if you have the money is the availability and the willingness of certain factions to sell them to you. the base bug mechs have their own category here. they can be found in reasonable numbers everywhere. then (in the 3025 setting) the factions have certain loot that they are willing to sell because they dont give these mechs to their pilots in many cases. the base charger is such an exmple for steiner and davion.
but almost economies in the battletech universe or directly or indirectly state-controlled. and in certain time frames houses are just not willing to sell the mechs because they dont have enough for themselves (in 3025 the houses had issues to fill their own ranks with battle-ready mashines; the civil war era made mechs hard to come by in what was the fedcom; in jihad era the houses even restarted production of primitives again to fill up their ranks. after jihad there was a massive demilitarization). the fluff gives many examples for that. so i would really take a good look at a mechs fluff and the time frame and consider the state the given houses economy is in. battle salvage (not functioning and too damaged to make it viable for the houses to repair) is usually what a merc unit gets. the old merc handbook stated that one of the most important "stats" of any merc unit was the ability to repair and to maintain the whole unit. when the brothers kell arrived on galatea to found the kell hounds, they did not hire ONE jock in the first months. just techs! they had good reason for that ;)
another options are heirloom mechs. an impovered scion of a family of mechwarriors might find himself in the position of needing money. so he sells daddies heritage. many options here, btw...
but pristine factory mechs should be very hard to come by in any area for any private person. unless having the appropriate contacts etc etc.
well... i hope, i helped you with my little excursion here.
have fun