Check?

Is a Stream Rusalka for a Greed Mammon a good deal? Not sure what either are worth.

Also, is a Blood Flame Colossus for a Seeker Quadriga a good deal?

I usually see Blood Flame sell for a tiny bit more than Seeker Quadria. At most, the difference is 7 EP to 2 EP, but their prices are probable closer to 5 EP to 3 EP. Either way, Blood Flame sells for more. That being said, it doesn't really serve any purpose for any combo team, so Blood Flame may drop in price. Meanwhile Quadria is the most important monster in the psycho creation team, so its price is probably pretty solid ad 2-3 EP.

As for Rusalka and Greed Mammon... I'm pretty sure Mammon is worth a good bit more. Neither one is very useful for any combo agility team, but Mammon has nice stats and a great skill, so the second a covert demon agility monster is released it will be a great monster for that team. Furthermore, Rusalka has the worst skill in the game (in my opinion) when evolved, and pretty awful stats too.