Similar to attached, both mantle and two side built-ins about 30" wide, with adjustable shelving - what should I expect as an average cost range to have these built as all custom built with solid wood, quality work?
That depends a lot upon your location. If in a rural area, the prices will be much cheaper than if you live in New York City.
Offhand, I'd say start at $3,000 for a relatively simple design and go up from there depending upon location and features added. There really isn't an upper limit. Fancier trim, lighting and similar options can add a lot to the cost of a project like that.
You could probably find a cheaper route if you searched for a lone cabinetmaker working from a garage but there are disadvantages to that such as a lack of insurance and long lead times.