Learn Italian: La qualità di questa marca è tanto alta quanto il prezzo. - The quality of this brand is as high as the price.

No, they don't change here. When tanto... quanto are used with an adjective (like alta) to mean "as... as", they act as adverbs and their endings never change. They only change their endings (to tanta, tanti, etc.) when they are used with nouns to mean "as much" or "as many".

Why does the adjective alta end in -a if it's being compared to il prezzo, which is masculine?

The adjective always agrees with the first thing being described—the main subject of the sentence. Here, the subject is la qualità (a feminine noun), so the adjective must be feminine singular (alta). It doesn't matter that the second item being compared, il prezzo, is masculine.

Could I use così and come in this sentence instead?

Yes, absolutely! You could say La qualità di questa marca è così alta come il prezzo. When comparing adjectives like this, tanto... quanto and così... come mean exactly the same thing and are completely interchangeable.

Does marca mean "mark"?