The Makos were bought during the best years of the a League, the peak of an unimaginably vast economy, and are some of the smallest Warships ever built.
At the time of the Exodus, all humanity had just seen their golden age tear itself apart, the core of human civilization was in ruins, nobody knew if the most powerful general alive was going to turn on everyone else, and the entire Inner Sphere was gearing up for a war that for many would prove to be literally apocalyptic. Honestly, I'd be surprised if the Lyrans would be willing to sell Kerensky machine gun ammo, much less any of the battlecruisers that are the pride of their nation.