Figured, I would create an off-topic thread since this forum unlike other forums doesn't seem to have too much off-topic conversation!
Logically many of the people I know who play wow, also play MTG so I figured this would be a good topic. I figured I would also take this as nice little opportunity to look at this game in a consortium / goblin-esque manner.
I figured I would start with some basic card valuing tools / sites
The Black Lotus Project - This is the Undermine Journal of MTG. This scrapes data from completed listings from eBay.
Magic Online Traders - Home of the famous MOTL (Magic Online Trade List) MOTL is also based off of eBay. This has a nice forum for buying / selling / trading cards. I've done it more than a couple (10 ish) times, haven't been scammed yet.
magiccards.info - Has a nice picture / text layout of cards as well as set listing. It also lists the lowest prices from RETAILERS. That info can be useful.
MTG Salvation - For your spoiler alerts, go here. It helps to know what cards might appreciate in value when new cards get spoiled, as well as what players are play testing.
Star City Games and Channel Fireball - These are the 2 biggest retailers online. They have nice strategy articles, and sometimes can lead you to cards that are pushing tournament playable and can appreciate in value.
MTGO - Magic The Gathering Online. You should have this regardless if you want to invest in playing online or not. From a game improvement standpoint, you might wanna play test stuff on here, or practice drafting (which is sweet since you can draft whenever you want). I'll explain below why EVERY player wants this app.
Auction Sniper - (PAID SERVICE, It's a clean link, not receiving any kickback on it) This will help you win auctions a lot more, for a lot less than standard bidding on ebay. Most players won't want to spend too much actual money on auctions though.
Quiet Speculation - Finance and strategy articles here. Great reads. There have been tournament level decks spawned here, as well as some really nice financial tips.
http://www.essentialmagic.com/ - Deck creation tool, credit: calianna
Magic Workstation (MWS) - An application that allows you to catalog your inventory, build decks, print proxies and play online. This is will definitely help you with budgeting MTG, since this is much more expensive than WoW!
Podcasts (credit: Mugsley)
Monday Night Magic
Limited Resources (Drafting and Sealed)
Yo MTG Taps! (tournament discussion and general interviews)
Scrubland (more tournament discussion; I just really like listening to these guys)
Basic BUYING strategies.
Here's a couple pro-tips on what I like to do when looking to get cards via CASH. My local currency is the USD, so all examples will be given in that.
eBay - do your research on what these cards are worth. Set up snipes and win the (hopefully) for the MOTL / Black Lotus Project price or less. If you want to know what you should NOT be paying, check out starcitygames.com. They're notorious for overpricing.
Magiccards.info - If you're looking for cards that aren't in heavy circulation for a variant format such as EDH (for those who don't know, Elder Dragon Highlander. It's a ton of fun!) check these sites, be prepared to get killed on shipping unless there are other cards you need from a certain seller.
Tournaments - Local small card shops will frown upon this, or it might get you kicked out! So be careful! In larger tournaments you can often find players looking to sell because they know that players come missing cards for their deck and look to make a profit. I personally have a friend who does this. He buys low and sells high. He is what most MTG finance people will dub as a "Shark". Watch out for these guys. I'll detail exactly what these guys look to do in the trade section below. If you are careful, you can get what you need for a reasonable value.
Magic Traders Online - You can often get good (ebay or better) prices from these guys. Another word of caution, this forum tends to follow market trends to the highest extreme. If a tournament deck has some super-secret tech and takes down a big event, expect those cards to be marked up 200 - 300% because EVERYONE will want them.