Yeah, the Rare cost of the latest upgrades feels a bit too high. It should be more like 100 Rares per upgrade. If they wanted to keep the current grind as is then I'd also be fine with them increasing the Common cost by 50 per upgrade. A marketplace to convert samples would be nice too, even if it's at a significant penalty (like 5 commons for a rare, or something).

It's an issue with the ratio of costs vs. the ratio of samples distributed. The total cost for all tier 4 upgrades is 1050 Common, 900 Rare and 100 Super samples, or a ratio of 10.5:9:1. From what I've seen the distribution of samples on the map is more like 14:10:1 (e.g. a difficulty 7 map might have 40 Common, 30 Rare and 3 Supers available). The Supers are easier to collect too since all three are guaranteed to be found at the Super sample rock. Arrowhead ought to have data on the ratio of sample types successfully extracted, and I bet that data would show that the ratio of Rares getting extracted is lagging the amount needed to keep pace with the other sample types in terms of the cost ratios.