A lot of people always ask that question - what MAC eyeshadows are worth investing in? While I myself do not own a ton (I currently own 10 with another on the way and possibly a few more to replace that I haven't gotten around to), I have narrowed it down to the few I own/will own (based on usage from a MUA) so that I don't waste my money. With MAC, it is key to:
a) always go pro - the quad (empty) costs $10 CAD (don't know about the larger version but I prefer the quads because it's easier to travel with and fits better in my drawer dividers - plus the price has gone down) and the pans cost $12 CAD + tax a piece (down $2 from last year, where I bought a pan for $14 CAD + tax), which is a few dollars cheaper than buying individuals.
b) make your money's worth - I've heard people say buy colour you use the most from the drugstore brands and buy "special" colours from MAC. Okay, there are drugstore brands that can compete with MAC and the price is less steep but in my opinion, if you buy MAC you will technically spend less because there is better pigmentation and you will probably purchase less just because you already spent on MAC and have the right colour (hence the need to choose wisely)
c) buy colours that can work with everything or a lot of the colours you own - when I first started, I got shroom, woodwinked and satin taupe. Why? Because I wanted a good highlighter and two colours that could work on their own and separately. On top if that, I knew that Woodwinked would work with the existing purples from one of my L'Oreal quads, as well as with my matte navy Revlon eyeshadow (I had asked the MUA too)
So here are the 10 eye shadows I currently own - note that I actually don't like one of them, but I'll explain that later.
(Top is without flash, bottom is with flash - these are my three quads (note that two pans are empty) but I only really use the two on the right more)
Colours are - from left to right going clockwise:
Goldbit, Humid, (empty), Li'Lily
Shroom, Handwritten, All That Glitters, Woodwinked
Blanc Type, Shadowy Lady, Satin Taupe, (empty)