Once the right variety of tree has been selected for the planting location:
The size/age of tree will influence the cost, there is always a size of tree to suit all but the smallest (<£1) budget. The price increases exponentially with size. At the larger sizes it is very close to the vertical part of the curve, upto the largest trees commercially grown.
Then you have the delivery/planting cost which again is a function of the size of tree, your location and planting location.
Once planted, please sit back and admire your investment.
Then the cost of aftercare, primarily watering, whilst the tree establishes (gets to the point where the root system has extended to that of a 'normal' tree and can look after itself). This period of time can last upto 5 years after planting.
It is a very false economy to invest all your funds in just the tree and planting. It will need assistance after planting.
The cost of aftercare can be reduced by (cool/damp springs and summers) automated watering systems or drip irrigation that you can plug your hose into. Mulching will assist
Please allow time (and therefore money) to care for the tree you plant.