The reason materia took a huge spike a few weeks ago was the 110 gear being introduced, and the prices are starting to settle down now.

For SB, the best way to get TIV is to have a SB dungeon party with some FC mates. Have a FC crafter make 4 sets of ilvl90 armor, stick a couple junk materia on each, then run the top 3 high level dungeons (the stone vigil hard tier). After 4 dungeons convert it all, each dungeon runner keeps 2-3 materia of their choice, and gives the rest to the crafter. Conversion rate on ilvl90 to TIV is around 75%.