I have a fireplace that is 20 years old with cracks now in back and side walls. Please advise how to repair and what kind of cost is involved?

Do the cracks follow the mortar joints or do they go right through the bricks at midpoint?

It is a solid concrete fireback not brick work

You might try calling a local chimney sweep and have them take a look at it and advise from there. Depending on the types of cracks that are there there are several different ways to repair them. Most is going to depend on access and how clean the crack is.

Concrete like a poured wall or is it a cement insert plate? Concrete is not a good liner for a fire place. All concrete cracks but the extreme heat of a fire will crack it even more, it should be lined with fire brick.