Thanks Spiky, updated it
I did the maths the other day, I think around 0.35% of users have the DIY Cake set - that surely makes it a solid Very Rare were it a pet. I think we'll see its value settle sometime next year when more people want it because they weren't here for the event. Or perhaps the year after because the people that join around now and next year will have more valuable pets to spend. Until then it's anyone's guess really.
The Cake set is much bigger and more interactive than the Chicken O's set because you can play with it a lot (you customise the cakes) and you can use it on multiple pets so it's really a group of items all in one. I'd say that the cake set will have a huge demand compared to the Chicken O's items simply because it's a customisable set rather than a single item. So while the Chicken O's stuff is a lot rarer, it's also a lot less in demand.
I think the problem with the Chicken O's stuff is that they are too rare, which drops their demand. They are very, very few of them, but people don't want to pay so heavily for them. So they are in this weird place where they are worth more than what people will pay, but people can't pay less because they are so extremely rare.
I think the UR Butterfly Wolf is a bit like this too, it's extremely valuable but it's demand is lowered because not many people want to pay that much for it.
In conclusion I have no idea how much the DIY Cake Set is worth. I've placed it above a low Sept 08 and below the Gearback Tribal, which is a massive range. I'd argue that it's worth more than a low Sept 08 dog because there'd be a more low Sept 08 dogs out there than cake sets (light leopard is rare tagged). That's assuming the rarity thresholds are where I think they are. From what I've seen the 'rare' tag has a huge range.