The only discount is if you qualify for the T-Mobile Advantaged Program which gives you up to 15% off service not phones or accessories. My Company qualifies so you can see if yours does.
Business Cell Phone Plans | Employee Discounts | T-Mobile Advantage Program
Other than that everything is pretty fixed. Carriers really don't make money on the phones they sell them only as a necessary evil so they can't really negotiated. As you pointed out the biggest part of your bill will be the EIP on the phones and if you do the math from the other carriers it will come out about the same or even a little less but then after 2 years they will keep charging you for a phone that you "paid for" where as with T-Mobile your bill will get cut in half.