I don't know what temperatures you expect to deal with, but I think you can come up with a more cost-effective solution (depending, I suppose, on how often you wish to use it). I would focus on minimizing loss of heat and using the heat present in the environment.
I think you'd benefit from building some sort of enclosure (open on the top, of course) to minimize outside airflow/wind taking away your heat. You could also, if desired, affix reflective surfaces to the enclosure to direct more sunlight in (though maybe not directly onto your person). If you made a clear top, you could allow sunlight in while you're not in there, which could better heat up the area beforehand. Insulation on the sides and the bottom would also help guard against heat loss.
Since you're sunbathing in this scenario, I would expect it to be sunny. So, you'd want to start by using a black surface to lay on, which could preheat in the sun. Further, you could fill some black jugs with water, which would be able to absorb sunlight and store heat in the water. These would go under you (or somewhere in there) to heat you and your sun tomb.
Finally, while noting the above, there are transparent materials that do permit UV to pass through them (at least mostly), so you could have a top made of such a material to further encase yourself. But I have no idea how much such a material would cost.
I suspect that a solution as above would suffice for temperatures at or above freezing, though I could not say how far below that it could manage. Part of it is up to a person's own tolerance. I guess you could just look into solar ovens, as I'm basically describing a person-sized version of one.
