I have been collecting over 5 years now. There was 
a time there were so many good cels on the 
market that I spent a lot of money. There is less of 
that today, but whenever I see a cel I want, I still 
go for it. My collection makes me happy, so it is 
worth it to me. Since I don't intend to sell, the 
monetary value is not important, except when I am 
the buyer. Since I have a large collection, I am 
tending to be more picky about the shows and cels 
I acquire.
Expensive (good quality) cels tend to increase in 
value more than cheap cels which are in plentiful 
supply, but there are no guarentees. Like the stock 
market, some cels are overpriced and adjust to the 
market over time.
Strictly from an investment view however, I 
suggest buying a house. According to current 
trends, it will almost surely give you a better return 
on your investment.