A good one from totalbeauty.com:
Luxe vs. Less Nail Polish
Red
Nars Nail Polish in Dovima, $16, brightens nails with the punchy color of market-fresh cherry tomatoes -- perfect for hot summer days. But Bobbi Brown Nail in Tomato Opaque, $8.50, gets the job done just as well for half the price. (The catch: Bobbi Brown polishes are also half the size of Nars bottles.)
Sparkling Pink
The only visible difference between Lippmann Collection Color Nail Lacquer in Don't Call Me Baby, $15, and Creative Nail Design Nail Lacquer in Translucent Tangerine, $6, is that the former contains a yellow-gold sparkle and the latter contains a white-gold sparkle. (CND's take, available in April, is also a tad glossier.) But as far as sheer, bright color goes, the two look identical.
Bare
We think putting nail polish in a pump-able pen is genius. But the gorgeous YSL Nail Touch Nail Lacquer Brush Pen No. 2 (available in May) costs a whopping $25. If you can sacrifice the luxurious gold packaging, Sally Hansen Color Quick Fast Dry Nail Color Pen comes in the same shade, Sheer Beige, and works the same way for only $7.99.
Copper
Estee Lauder Pure Color Nail Lacquer in Bronze Goddess (left) will give your nails the glowing tan of a Brazilian beach goddess -- for $18. But L'Oreal Paris Nail Polish in Goldy Lucks can do it for less (only $4.59). Take your pick!
Rose
You really can't go wrong with a romantic rose polish. The classic Lancome Le Vernis in Beige Elegance (left), $18, looks a hint more brown than Zoya Professional Lacquer in Penelope, $6, but the miniscule difference is well worth a savings of $12.
24 Karat Gold
We thought we'd never find a match to Shu Uemura Nail Enamel Supplement Mineral in Gold Zest, $16, a beautiful, shimmering light gold. But OPI's India Collection came through. Curry Up Don't Be Late!, $8.50, is a near-exact copy, sans seaweed extracts and the "tri-color confetti pearls" that give Shu Uemura's gold a subtle kaleidoscope effect. (Both polishes are formaldehyde- and toluene-free.)
Plum Shimmer
From time to time, brands launch limited edition colors like MAC Nail Lacquer in Boom!, $10, which sell out almost immediately. No need to panic in this case. Sephora Brand Nail Polish 52 (right), $4, comes in a smaller bottle but looks almost exactly the same after two coats. (Shhh.)
Blues
Despite treks to stores throughout the city, we couldn't find an inexpensive, exact color copy of Chanel Le Vernis Nail Colour in Blue Satin, $19. OPI Nail Lacquer in Russian Navy, $8.50, was the closest match that was still available on shelves, but it wasn't quite blue-black enough. All hope is not lost, though; Dior Vernis in Poison Blue, $19, is a near perfect match, lacking only Blue Satin's virtually nonexistent shimmer. The catch: It's a limited-edition color, equal in price to the lovely Chanel shade. (Alas, some things never change.)
) I find that while Wet&Wild is a great value for your buck (literally) when you want a trendy nail polish color, they just don't last, even with a good top coat. And with Chanel polishes, I feel bad spending $20 or more on a polish since I realize that in less than a year, it'll be all goopy and dried out, even if I keep it in a fridge. For me, OPI is the perfect middle ground - I think the polish quality is great, the brush and application is phenomenal and they do have a good amount of color variety. 
Very interesting.
I've found that the Sally Hanson and Maybelline Salon Finish are just as good as the more expensive brands I've tried like Essie and OPI.
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“Fashion fades, only style remains the same.”
—Coco Chanel