Hi
Apple's current prices for iPhone 7 Plus are available via the Apple Online Store. In the US, for example:
Buy iPhone 7 and iPhone 7 Plus - Apple
In some countries (including the US), Apple offers a trade-in program for iPhone. This offers instant in-store credit towards the cost of a new iPhone (if trading in at an Apple Store) or a Gift Card (if trading in via mail) when trading in an eligible iPhone.
In the US, for example, the estimated trade-in value of an iPhone 6 Plus is currently $170.
For more information (in the US, for example), including a link to get a tailored estimate based on the condition of your iPhone:
iPhone Trade-up - Apple
Another option is to sell your iPhone 6 Plus privately and put the proceeds towards your new purchase. If doing so, be sure to follow the steps here, including in order to remove Activation Lock:
What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support