Joust BE701- 1.65 mcb, 5/8, ERLL for 10pt hit, LRMs for direct/indirect, AP weapons, and 8t FF armor
Mithras- .87 mcb, 6/9, UAC/2, 2 ERML (1 in turret), 3.5t FF armor
Double the cost true but . . . well over twice the armor, 5 (or 2.5 in ultra) times the ranged punch roughly before entering LRM range, anti-personnel weapons, and the main gun is ammo independent . . . all for a bit of speed loss.
If you are not building Enyos, its the best alternate.
Btw, the ERLL version is a Jihad variant . . . which means they do not have it, if you are going to suggest the Shark/Foxes are not trading with them. Even supposing the ERLL, you still end up with comparable firepower (ERLL x2 vs ERLL & LRM10) at range with the Joust being better up close.