While Sprint gives a $150 credit towards a new phone with a 2-year contract extension it's monopoly money. On the one hand, their base prices are often higher than what other vendors charge. On the other hand, they sometimes offer additional discounts and rebates totaling more than $150. Your best bet is to check the Sprint store price on sprint.com and compare it to what other retailers are charging for the same phone.
Example Sprint Photon 4G
Sprint Price $549.99
Contract Disc.-400.00
Rebate -50.00
Net cost $99.99
eBay (no contract) $400
Radio Shack w/contract $49.99
New phone pricing falls into three categories:
1) Phone, no plan
2) Phone upgrade for existing customer w/new contract
3) Phone w/new contract for new customer or new line.
You'll have to read the fine print on the Nebraska Furniture Mart deal to find out which category the $299 price falls under. If you extend your contract with a third-party you may have additional ETF fees to get out of your contract.
Cut to the chase - what phone are you shopping for?