Rocks is probably the hardest card to accurately rank. First, we need to understand how valuable Catapult is. Then we need to understand how likely the Catapult pile is to be depleted. This will help us determine how often Rocks might even be gained when it appears in the kingdom. Then, in those kingdoms, we need to evaluate how useful Rocks are.
Here's my initial attempt to do this:
Proposition 1: Catapult is likely to be bought as the only Curse trasher in ~33% of kingdoms*.
Proposition 2: In most of the kingdoms specified in Proposition 1, providing Curses via Catapult will be so valuable that it will be worth it to buy extra Catapults to get to Rocks**.
Proposition 3: In most of the kingdoms not specified in Proposition 1, no more than four Catapults will be purchased, and Rocks will not be available***.
Proposition 4: In kingdoms where Rocks can be gained (mostly those specified in Proposition 1), they will be quite valuable - offering immediate economy and three sources to provide a brutal double attack. In these kingdoms, Rocks will be about as valuable as Ill Gotten Gains in kingdoms with weak curse trashing (Rocks are a slower, but more impactful attack with slightly better economy). In the presence of gainers like Workshop, they will be even better.
Conclusion: Rocks will not be obtained in ~2/3 of kingdoms it is present in. Rocks will be quite valuable (~7/10 $4 card?) in ~1/3 of the kingdoms it is present in. I think Rocks compares favorably to Rats and Feodum, which are also situational cards that are quite valuable in a minority of kingdoms.
* I calculated this value by assuming that Catapult will be purchased as the only Curse trasher if it is not in the same kingdom as: Chapel, Masquerade, Steward, Upgrade, Replace, Ambassador, Lookout, Salvager, Apprentice, Expand, Remake, Jack of all Trades, Trading Post, Forager, Hermit, Urchin, Rats, Junk Dealer, Altar, Butcher, Ratcatcher, Raze, Amulet, Plan, Trade, Temple, Donate. This is a sloppy assumption, but I think there is sloppiness on both sides (sometimes, Catapult won't be worth pursuing in the absence of these trashers; sometimes, it will be worth pursuing as the only Curse trasher in kingdoms with these trashers).
** For an argument as to why this is, see
here. However, I haven't played many Catapult mirrors. It is possible that there is a stable equilibrium in a two-player game where each player has two Catapults, and neither player wants to purchase the last Catapult and expose Rocks to their opponent. Theoretically, additional gains or buys could break this stalemate, but it will be challenging for a player to gain a Catapult and a Rocks in a single turn while being attacked by their opponent's Catapults. If this equilibrium is common, I will have overestimated the value of Rocks. <philosophizing>Or, does this equilibrium imply that Rocks are valuable despite the reality that they will rarely be purchased in a game with two skilled players. I don't believe there is a similar situation in Dominion.</philosophizing>
*** The biggest exception to this proposition, I believe, is the presence of a gainer like Ironworks. If a player can gain extra Catapults cheaply (trashing the duplicates), then it might be worth it to try to reach Rocks. The gainer will also make Rocks more valuable - in addition to easier to obtain.
Edit: Because English is hard.