With the 4S, the cost is dependent on which model you get. They run $200 (16GB), $300 (32GB) amd $400 (64GB).
That's if you are upgradeable and you sign a new 2 year contract with your provider (and you purchase the phone from that provider). Just purchasing the phones outright is MUCH more expensive, like starting at $650.
Wait...what?! I can't to that. That is way too much!
Here's something else to think about...
While I personally think you should let him by the gift he wants to get you and the one you ultimately would like, if spending $ on you is something that you will always be uncomfortable with, then you need to tell him.
This is a committed relatationship, and I'm assuming one that at the very least, has a great potential for lasting for a very long time. If you talk yourself into accepting this gift now, and he continues to want to buy you extremely expensive gifts in the future, then you'll always be uncomfortable....and that could strain the relationship
So, if you can't get to the point where you're ever comfortable with letting an SO spend large sums of money on you (as long as he can afford it), then you need to explain things to him and work out what does make you comfortable.
Again, I think you should accept the gift. This isn't a passing relationship
. But, if when push comes to shove, you just aren't comfortable, then make that clear now.