I agree with the other guy. Car audio is VERY tricky. What i tell people is that, you will be spending more than you anticipate. There is a lot more that goes into subs than people think. I'll try to do a rough price breakdown for you...
2 10 inch kickers with a box NEW will run you about 200-250. You will then need an amp. I recommend purchasing your amp at the same store you buy the subs because they will help you to match the amp with the subs. DO NOT BUY A CHEAP AMP. if there is one thats $100 and is a brand like legacy or pyramid, and one that is memphis or jl audio for $200 with the same specs, go with the more expensive one.
Also your gonna need a wiring kit. Memphis has complete kits with everything you need. The cheapest ones are about $50 and the most expensive $100. Again, i recommend the more expensive one because if you ever want to upgrade down the road, chances are you will need large wiring to handle more power. Just do it right the first time and get the best wiring you can. That way, you will never need to re-wire.
Last but not least, install. Most sound shops are going to charge at least $50- $60 an hour to install your stuff. If you have the cash, let them do it because it will save you a lot of headache. However, if you choose to do the install yourself, have a couple friends who know about wiring help you out. It is a good learning experience, although a very frusturating one.
So, you should expect to pay about $400-$500 if you do the install yourself, And $550-$750 if you pay someone to do it...... Good luck my friend