Oh boy. This is going to be a long response.

Why is it better than supplementation

I already went into this in a different response, but I'll repeat myself for you here.

Firstly, because supplementation has downsides. Vitamin D is made from cholesterol, so it has an effect on the cholesterol in your body. Supplementing with exogenous vitamin D can raise levels of bad cholesterol in your blood, in contrast to using your skin to produce vitamin D which lowers the levels of cholesterol in your blood.

Secondly, sunlight exposure has many benefits which have nothing to do with vitamin D. So you won't get them from a pill. UV exposure raises levels of nitrous oxide, which causes lower blood pressure, eposure to blue light reduces fat storage, exposure to bright white light reduce depression and seasonal affective disorder, exposure to violet spectrum light can prevent myopia, etc. Those are in addition to the benefits associated with vitamin D production.

I built my lamp with the intent to maximize the benefits that can be obtained from natural sunlight, to the best of my ability. It isn't laser focused on just maximizing vitamin D.

how much cheaper is it than something pre-made

That question is very difficult to answer because (to the best of my knowledge) there aren't any premade products that aim to achieve the same results.

Most "sunlight" lamps marketed towards treating seasonal affective disorder contain zero UVB and zero deep red and infrared light. They're just bright white lights.

There are some lamps, such as the Sperti lamps, that do contain UVB and that can cause your skin to produce vitamin D. However, they aren't full spectrum lights so you miss out on all the extra benefits that I mentioned earlier. Also, they only expose small sections of your body at any one time, whereas mine can hit your whole body at once.

Sun tanning booths contain HUGE amounts of UVA which cause premature skin aging, and once again they are not full spectrum. You'll tan, and you'll produce vitamin D, but you pay 3x as much money as you would for my lamp build and you still miss all the extra benefits that I listed.

There are reptile bulbs that you can buy off the shelf, but their spectrum has gaps, many of them can contain UVC wavelengths which can burn human skin very quickly, and the fluorescent versions also don't have any deep red or infrared spectrum. The mercury vapor or metal halide reptile bulbs will contain some deep red and infrared spectrum, but they consume a huge amount of power for the light that they emit. For example, see this bulb on amazon which uses 160 watts for a single bulb. The UV level emitted becomes practically useless after just a few feet, so you need a large array of them. The completeness of the white spectrum and specifically the near-uv/violet wavelengths leaves a lot to be desired as well.

So my question to you is this; what "off the shelf" product do you even try to compare to my lamp?

and is that cost saving mostly due to the lack of red spectrum?

The question is irrelevant. Some DIY projects are intended to save money. That is not an intended goal of this project. My goal was to simulate sunlight as closely as I could, and I did it specifically because there were no off the shelf products that are comparable. I created this setup because it was necessary for me to build it from scratch, not because I wanted to save money.