Think of this sorta like trading cars. New cars seldom sell for the sticker price, do they? And you are seldom offered what your old car will actually bring on the used market.
Not being able to see your Glock, or to know the local market for used pistols in your area, it is pretty much impossible to say that this is a "good" or "bad" deal, but I can say that you can probably do better by selling your old gun yourself...if it has 15rd. mags, this is a real plus for a sale...and shopping for a lower price on the Springfield for cash. This, however, is going to take time...instant gratification is more expensive.
Like cars, you "shop" for a gun by being willing to walk away from a deal if it is not what you consider to be fair. If you decide you just GOTTA have THIS gun right NOW, be prepared to pay more. Also, don't fall into the trap of believing that gun dealers are ALL out to "fleece" you because they try and make a profit. They have business expenses such as insurance, rent, salaries, taxes etc. etc. etc. that must be covered, while a private seller has none of those considerations. Yeah, some dealers are assholes...but then, so are some customers!
Good Luck