A very small percentage of F150 trucks have the base 3.7 V6. Almost all come with the 5.0 V8 or the 3.5 EcoBoost V6. The cost of the upgraded engines depends on which trim line you are buying. The Lariat comes standard with the 5.0 and the Eco is less than $1000 more (it was a $700 option when I bought mine in 2011). I forget which engine is standard in the XLT trim, but if it is still the 3.7, then a upgrade to the 5.0 would be less cost than buying a 5.0 in the base XL trim line. Often it is to your advantage to go up a trim level than to buy a bunch of options and still get the lower trim.
Either way, having the base 3.7 in a truck will hurt you when it comes time to sell it or trade it in down the road.
Also, watch the Max Tow options on the truck. Ford is calling the towing option Max Tow now, and it really isn't the best towing package available, as the cargo rating is less than 1700 pounds. The big towing package will be over 2200 pounds, and it will come with 7 lug wheels and the nice tow mirrors, along with the equipment underneath you need to get that big rating.