If you are buying an immersion wort chiller there are several factors that you need to be aware of. The type of metal used to construct the chiller is important, but the length and diameter of the pipe used have the largest effect on the cost and efficiency of the chiller.
If you search the web you will find immersion wort chillers for a variety of prices, usually ranging from $35-$100. It is important to know what you are getting for these prices (Note: this comparison uses prices for copper immersion wort chillers).
50-foot long immersion chillers tend to sell in the $100 range, 25-foot chillers around $50, and there are some chillers for $35 that work out to be around 18 feet long (a lot of these don’t actually list the length so this is calculated from the dimensions of the chiller). These may be constructed using any diameter of tubing, but usually 1/4″, 3/8″, or 1/2″ are used. Changing the length or the diameter of the piping affects how much surface area of the tubing there is to exchange heat with the wort, thus affecting the efficiency of the immersion chiller. The chart below shows the change in square feet of metal in the chiller with a change in diameter or length:
Most immersion wort chillers that are found on the market in the $50 range are 25-foot chillers constructed with 3/8″ tubing. The cost per square foot of surface area for these chillers works out to $20.38. The same is true for 50-foot chillers made with 3/8″ tubing, which typically cost around $100. They also work out to $20.38 per square foot.
The interesting note to take away from this is the value that you get for chillers with different sizes of tubing. The cheapest immersion chillers on the market sell for $35. They are 18 feet in length, and they use 1/4 inch tubing. The cost per square foot of surface area? $29.72. This shows that smaller immersion chillers are more affordable but you are paying more for a smaller chiller relative to its efficiency if the tubing is 1/4 inch instead of 3/8 inch.
On the other side of this, there are 50-foot immersion chillers on the market that sell for $100, even with 1/2 inch tubing. This comes out to $15.29 per square foot of surface area. This is a pretty good deal!
The size and efficiency of the immersion chiller for you ultimately depends on your specific brewing setup and your desires. When you are selecting a chiller, though, keep in mind what you are paying given the efficiency of the chiller.