I've bought most of my decks used, for cheap.. under $20 for most. I think everything that was mass-produced eventually turns up in a secondhand bookshop, or slips under the radar on Ebay eventually, so if you're patient and vigilant, you can even get your hands on OOP things for very little money.
In the last couple years, there have been 4 exceptions:
The first deck I ever spent what I considered to be a lot of money on was a large Illuminated Tarot from Carol Herzer, with irridescent paint added. I paid $180 for it.. and remember being terrified that it would get lost in the mail, that I wouldn't really like it, or that the cards would fall apart.
Unexpectedly, it became one of my primary reading decks, and has remained such for several years.
I think that not only is it a great deck, but also one has to remember that the artist put many hours of work into creating it... I think supporting people who create beautiful things is a great thing to do, even if it means having to save up for a few months to do so.
A few months ago I bit the bullet and bought another deck from her.. one with heavy glitter and irridescents painted on, for about double the price. I couldn't really afford that, and am still paying off the credit card as a result. Probably worth it... because.. sparkly!!!!
About a year ago, I spotted an antique deck on ebay France that I had always wanted but suspected I would never have a chance to own... a Grimaud Arnoult (1891! wow!) Some money had appeared unexpectedly around that time, and I took it as a sign that I should put in a bid. I couldn't believe it when I won!!
http://www.flickr.com/photos/45958274@N04/sets/72157629475901350/with/6942061182/
I paid $350, including insured shipping from France. I justified it with the age/rarity of the deck, and also that it was one I'd had my eye on before.
I have never been so excited to get a deck in my life!... but it's killing me because I've had to wait more than a year to actually see and hold it. I don't ship expensive things to Uruguay because of theft/loss/delays in the postal system and high import taxes. Instead, I send things to a friend in Canada, and she hangs on to them until I have a chance to travel and get them in person.
Fortunately, my husband is travelling soon, and returning in mid-May with my precious cargo, so I don't have to wait too much longer. I feel a little guilty that I think I will be more excited to get my hands on my cards (this deck, the above sparkly, the one I'm about to describe, and a couple others) than on the hubby that I'll have then gone 3 weeks without seeing! :S
The only other expensive deck I've bought was a 1929 J.A. Knapp.
I think it was a steal at $100, even though the condition is not very good (but that just adds character, in such an old deck!). I have rarely even seen this deck for sale... I always thought I'd eventually get the OOP USGames version, and had been keeping an eye out for it for years. It was an impulse buy, but I'm quite excited about it anyway. No regrets!
http://www.flickr.com/photos/45958274@N04/sets/72157630863251628/
I have also -almost- bought the Thomson Leng about 4 times and been outbid in the last seconds... I'm sure that will be another deck I'll end up spending too much on at some point in the future.