I find that online, anyway, Small games are considered at about 8000 BV, Larger games at 15000.
However, I find that building a balanced lance should sit around 6000 or so BV. 8000 only if you're using C3 networking, because C3 tends to get overcosted. If you want to get a good idea of where your lance will sit, think of the lightest weight you'll accept in your force, and the heaviest. Divide the sum tonnage by two. Now pull out a TRO and look at some of the 'mechs in that tonnage area. Pick one (According to what TRO you plan to pick from, BV can vary. 3025'ers will be considerably cheaper than 'mechs from 3085, for the most part.), and multiply that value by four. There's your ballpark lance Battle Value. Remember that if you push it to the lighter end, you'll be able to boost your mechwarrior's skills to compensate for other players. Likewise, if you're going heavier, you might be forcing the other guy to increase HIS pilots' skills, or pad out his army with extra units, like Battle armor, tanks, etc.
Also, It'll be handy to keep at least one unit that sports standard SRM launchers. Nothing nullifies the threat of combined arms forces like a bin full of infernos. If you're not familiar with Inferno SRMs. Take a look at them, and how they affect each different kind of unit. Against 'mechs, they're doorstops, but against most everything else, it's saving grace.
But don't forget that you can toy with your piloting skills to balance a lance out. That's a course of action I often use.
