I've had quite a few reader's requests after my original Chanel Mimosa review here so I thought this might help you find an alternative, on the cheap...
I don't wear that much yellow nail polish but I do have 3 shimmers to compare Mimosa to.
I'll start with the least similar and end with the most approaching.
Kiko 269 Microglitter Yellow
Kiko is an Italian cosmetic company, I already introduced it here.
Microglitter Yellow is actually more a glass flecked nail polish, rather on the sheer side but with an excellent formula, I needed 4 coats to reach this level of opacity but I think it works better layered, even with 4 coats, there is VNL (Visible Nail Line). It's very sparkly and if you wear this in summer, you might want to be careful or you might end up blinding someone ;-)
H&M Going Bananas
This nail polish is quite new, it's from the Spring 2011 H&M nail polish collection, it's a pale yellow with green and orange shimmer. Apart from being lighter than Mimosa, it's also cooler, most certainly because of the green shimmer.
Formula wasn't too bad but it wasn't dreamy either, you have to apply 3 thin, careful coats for this result.
Catrice Don't Feed the Birds
Catrice is a European brand, I was lucky enough to get this nail polish, along with a few others, from a friend a while back because I didn't have access to it but was lemming it badly.
Don't feed the birds is a bright shimmer yellow, again, quite on the sheer side. The shimmer is much more dense than on Mimosa and the shade itself it a tad cooler but it's definitely a good substitute for it, given the price, around $4 for 10ml / 0,34 oz.
On the downside, I needed 4 coats here with VNL, but they applied smoothly, I also tried layering it. I used one coat of H&M Going Bananas and one of Don't Feed the Birds and the coverage was perfect this time.
In conclusion, I'd say that if you're not ready to drop the $$ on this summer's new Chanel nail polish you'll definitely be able to find something to satisfy the urge for yellow.
I also guess is it won't take long before dupes start to pop up everywhere...