Is it worth getting 10 inch subs instead of 12's since 10's are cheaper?
The major difference you should be looking at is the difference in sound. I have three Alpine Type-R 10-inch subs, myself, and I prefer them for the power and resonance. With 12-inch subs, you're going to get a sharper and cleaner bass hit (great for stuff like metal). Another important factor is the fact that with higher wattage and more subs, you can get a very nice bass sound out of a sealed box as opposed to using a vented box for the increase in presence. When it comes down to the price difference, you should disregard the extra couple of dollars you'd spend on 12-inch subs if they produce the sound you want in your vehicle. You're already spending the money on the subs, you may as well get something that sounds how you like it rather than go with something that you won't enjoy as much to save a few dollars.
*I forgot to specify that I was talking about making a very clean and clear bass hit with 12's in a sealed box. I just realized that what I posted about the box seemed off topic. My mistake.
I wouldnt worry so much about the cheaper size of sub, the difference in price isnt too drastic until u get to more pricier subs anyways. it really depends on the kind of music you listen to really... simple rule is if your listening to country, or heavy metal.. 10 inch subs will probably sound the best, and if you listen to alot of rap with low bass youll feel it alot better with 15s. but 12s fit nicely in the middle. so its really up to you, but remember, to get the most performance out of your system make sure you build your box for the subs you buy... they take specific cubic feet of airspace. good luck on the build.
go to a good professional audio store. not best buy or something similar to that. they will help you find exactly what you are looking for. some 10s are better than some 12s. its all dependant on what you want it to sound like and thats something that most here cant help you with.