The plan isn't more expensive, it's just a financing option. Think of it as a lease with a car (same thing...this is a lease but with a phone). Basically they split the cost of the phone into however months your plan allows (either 18 or 24) and that's what you pay per month (some devices might have an initial down payment and you will have to pay the sales tax for the full price of the phone on your first payment). Now, since this is a lease, you don't OWN the phone, T-Mobile does. Same as with a standard lease, should you choose to keep the phone, you can pay it off or do the purchase fee at the end. With me so far? OK.
So what does the JUMP option do? Magic. It allows you to switch phones every month if that's your thing. On a standard lease you'd trade in your device after a certain period of time (usually until at least half the device has been paid off). Here, you can do it up to 12 times a year with 30 days in-between 'jumps'. You trade the device in, the remaining payments are canceled, and you walk out with a new device (but also a new lease, so your 18/24 month counter resets every time).