Unfortunately, you are talking about two variant rules from two separate books, that are technically in separate editions (kind of; Unearthed Arcana is definitely 3.5e, while Savage Species occupies a strange 3e-3.5e hybrid space as it was published while 3.5e was nearing completion). How they should interact was most likely never considered by the authors of either, and the rules themselves definitely do not discuss it.
But Savage Species monster classes generally did not aim to make LA less burdensome once you got it, it merely aimed to make races with LA playable at levels too low to have so much LA, so you could gradually grow into your LA (and, supposedly, commensurate powers). As such, there is a pretty simple proposition:
If LA buyoff is balanced for a given race, buying off the LA for that race when that LA is gained via monster class is just as balanced.
The only important point is that you would have to consider the total LA of the race, not the partial LA you have for being partially through the monster class. So no buying off the first LA +1 of an LA +3 race as if it were only an LA +1 class. This is because Unearthed Arcana considers the LA of higher-LA races more valuable, on a per-LA basis, than those of lower-LA classes. So if you have a monster class that gives LA +3 eventually, but has only given LA +1 by 3rd level, you could not buy if off at 3rd, as if the race were LA +1 total. You would have to wait until 9th level to do so, per the table entry for LA +3 buy-off in Unearthed Arcana.
That is, if we are trying to be faithful to Savage Species and Unearthed Arcana, presuming them to be balanced.
But in reality, this is not even remotely the case. LA was poorly designed to begin with, Savage Species was originally a completely-separate spin-off product line that got canceled and its existing material forcefully shoved into D&D even though it didn’t really fit, and Unearthed Arcana is basically throwing a bandaid over a gruesome laceration. None of the above material is really well designed or truly “balanced.”
Worse still, attempting to fix that problem is beyond the scope of this answer. It’s actually probably too much work to really be fixable at all. In my games, I simply do not allow LA to exist. I hate that answer, because I would like for these kinds of creatures to be playable, but the simple fact is that the game does not work well with them as PCs. When possible, I try to develop LA +0 versions of LA’d races for players, but this is difficult for low-LA races, and completely impossible for creatures with more LA than +1 or +2.