I built one last year to help me move all my equipment and machines to a new house/shop. One of the best investments I ever made. Cost was about $500 (CAD) in materials including casters and an I-beam. About half the cost of the cheapest machine rigger I could find.
The A-frame is 4" square tubing with a 0.25" wall. The uprights telescope inside the A-frame and allow for a lifting height (bottom of I-beam to floor) between 7 and 10 feet, adjustable in 6" increments. I've seen some that use hydraulic cylinders to raise and lower each side, but I designed mine to be able to raise and lower using a floor jack under the upright. I do one at a time and it works beautifully.
The I-beam is attached to the uprights using custom made clamps, which allow the width to be varied as well. This is particularly good for really heavy loads, as my A-frame currently out-strengthens my I-beam in terms of lifting capacity at full width. I-beam to be upgraded at some point in the future...
Pictures aren't the greatest, but you get the idea. Don't mind the single lifting ******** the workbench - I wouldn't normally do this, but it was one of the last items to leave the shop and I had left all my straps at the new house on the previous trip.