Build an Army on the Cheap: 3-Step Method
Let's face it, Wargaming is an expensive hobby and it isn't getting any cheaper. In a bad economy in which the cost of everything is rising and salaries are staying the same, it's hard to justify starting a new army. If you buy retail, there are very few (if any) armies you can start on a budget. There are, however, some different tips and tricks you can employ to make the hobby more affordable. The three-step method listed below is my standard protocol when trying to acquire new models / armies. Hopefully others can get some use out of it as well. Step One: Search the trade forums (Dakka Swap Shop, Bartertown, etc...) and eBay for army deals. If you have specific models in mind, try finding deals with the largest amount of desired models included. Try to find people getting out of the hobby, or who will give you a discount for buying in bulk. The lot should be somewhere around 50-75% off the retail price.
Step Two: Sort out what you want, and what you don't want from the large lot. If the models you wish to sell are in poor condition, it may be beneficial to spend a little time cleaning them up by priming over bad paint jobs or (if you have the time / patience) giving them the Simple Green / paint stripping treatment.
Step Three: Sell the remainder individually on eBay, Bartertown, Dakka Dakka, etc... for 30-50% off retail value. If you have enough models left over and are short on time, you may sell the remainder as a single army as well. My preferred method is Bartertown or other forums. Trying listing your lot for your desired price. If it doesn't sell in a week, try reducing the price 5%. If it's still not selling, go down 10%. If you can't sell it at 15% below your desired price, go to eBay. eBay should get you the best price out there, but they will take approximately 13% of your sale via listing and PayPal fee's.
Most recently, I did this with my Dark Eldar. Before the announcement of the new Codex came, I found some HUGE army deals on Bartertown and Dakka for between 65-70% off retail. I then sold a chunk of it on eBay for just 35% off. The portion I sold contained Raiders (some damaged, some in good condition), a converted Ravager and models I already had plenty of. In other words, it was nothing I would miss. When it was all said and done I paid just over $500 for 7,000+ points of Dark Eldar (in good condition). If I were to purchase the same army through a storefront, I'd have spent $500 on just the 11 Raiders and 3 Ravagers (which, by the way, accounts for less than 1,000 points of the army).
If I desired, I believe I could sell $500 worth of my Dark Eldar and still have enough models for an army left over. What does this mean? There's potential to play Warhammer 40k for free if you don't mind turning over armies, spending time working with eBay and searching for great deals.
Now, not every transaction will be so beneficial for the buyer. In the past, I've made some miscalculations on the resale value of miniatures and struggled to get a good return. It's important to be realistic about what you think the resale value is on some miniatures, as some are in high demand, while others are not.
Practice makes perfect. Try scouring the trade forums for some good deals. Imagine how you would dissect the army in order to make a good portion of your money back while acquiring some models on your wish-list.