I remember the "Transmetal-palooza" you referenced. Before I knew it, I had over 600 of 'em. It was great, but it still cost over 100 to max one UR card, so it doesn't last long.
For the record, the number of transmetals it takes to upgrade a full R6 card exceeds 1000. I didn't get a good look at it today but as I went to max out my trans-scanned MotorMaster R6 card, it took OVER 1000 transmetals to upgrade to level 115 (the maximum). No joke, no exaggeration. It was after fully upgrading that R6 card (T1 prize for this last event) that I finally dropped back under 10,000 transmetals. I had been hoarding them, strictly using cards as fodder for my 90% of my upgrades the last 3-6 months.