Actually if he can finegal an invitation and token from a fellow member of your covenant, he needent worry about your ageis at all, until you realize he is there and disinvite him...
... which explains why covenants tend to be picky about their membership.
Of course. But, you only need one Aegis per covenant. So, spreading the cost out over multiple magi is exactly why covenants exist. Maintaining an Aegis in excess of 30 certainly looks expensive if you are only considering 1-3 magi, but if you have a covenant of a dozen magi it is much less of a problem. Also, each individual covenant doesn't need to harvest exactly the right vis itself, as there is a whole Tribunal network that you can utilize to trade vis and services.
The alternative to a high level Aegis is to have a high Parma Magica Score (say, greater than 6). And a high Parma Magica Score is very expensive in terms of magus time. The cost of a high Parma Magica Score is not so bad for "active" magi (like many PCs), because it is also available when you are out-and-about-and-kicking-ass. But (more mainstream(?)) magi who largely live and work within the confines of friendly covenants, would probably rather spend the time on improving their Magic Theory, or improving Art Scores, or inventing spells/items.
Having said all that, even a low level Aegis (Lvl 20-30) is still very useful. Because even if it happens to be too low to benefit the magi directly, it will still cover the covenant buildings, and grogs, and apprentices, etc. You also don't need to have a high level Aegis all the time. If you are vis poor, but otherwise capable of casting a high level Aegis, then a possible (albeit risky) strategy is to only routinely cast a low level Aegis and then cast the big Aegis on special occasions, such as Wizard War declarations.