Availability of high level labtext is like virtus source: very Saga dépendent.
I agree with Ramidel on all the points he made. I would also add that considering that House Mercere has a dozen of magi (I believe it is mentionned either in the Core rulebook or in HoH: True Lineage), it is not impossible that there might be occurrence where the labtext is available in the house, but there is (temporarily) no mage qualified enough to use it. He might be part of the Cult of Heroes, have access to the labtext, but short on either of the Art. Considering that the House needs also to maintain a Terram specialist for the Portals, plus a few Mutantum mages, it seems that if you are born a Mercere magus, you might be pretty limited in your career choice if the House wants to maintain its secrets.
In general, in my campaign, any labtext above level 30 is not automatically available. My PCs cannot just ask any RedCap for a labtext and have it delivered the next season. Some are more easily accessible (Conjuring the Mystic Tower, Aegis, Healing spells on top of my head), but other needs some personal research and contacting specific covenant or magus to have said labtext. It is usually not a matter of cost, but more of Relationship and trading favours.