Handy man or a small sole proprietor contractor/carpenter should be perfect for this.

If construction is continuing in your area I'd talk to one of the rough crew guys about a weekend project. Get the info on the door they would have installed. You can buy that and framing lumber. Ready to go.

and what is the average cost?

Cost will depend on labor and materials -- door might be $150 - $400, brick/stone exterior will be more than other materials. Will you need a concrete pad? Trim inside and out.

Could easily be a $1,000 project.

Don't forget if you have a stem wall sticking above your slab, you need to cut through that, otherwise you'll have to step up over the door threshold. Also better to remove a portion of the slab to extend the slab through the door. Mo' $$.