It seems that Sigh-Ray offers a 15-stop 77mm ND filter for $480 USD. I understand quality glass is expensive but that seems extreme. Is there anyone else out there who offers a 12 or darker stop 77mm filter at a more reasonable price. Stacking filters is not an option because I shoot mostly at the widest angle possible and when I stack filters vignetting is extremely pronounced the top ring is visible in the photo. I am also not looking at any Lee-like rig with square filters because they are cumbersome, expensive and take up a lot of space. A simple, low profile screw-on 77mm filter would be my preffered option.
It's easy to lost perspective when referring to stops due to the fact that it is a logarithmic scale. When talking about ND filters each stop represents a halving of the intensity of incoming light. Like grains of rice on a chessboard it starts to add up very quickly.
A 15 stop ND filter reduces incoming light intensity by a factor of 32,768. That's enough to turn what would be a hand-holdable 1/50s exposure into well over ten minutes. This makes it a pretty specialist piece of equipment.
Given the specialist nature of a 15 stop filter (which is therefore manufactured in small quantities, unable to benefit from economies of scale), along with difficulties in actually producing such a dense filter that doesn't result in a strong colour cast it is unsurprising to find one with a $450 pricetag.
If you really need to be able to do ten minute exposures in broad daylight, then a more economical solution would be to pick up a 10-stop ND filter shoot 30 exposures of 20 second each with an intervalometer, and then stack them in software. As a bonus you'll also get a boost in dynamic range!
If you want a real budget solution you could try welding glass, one example here, to give you an idea of what to search for: http://www.amazon.co.uk/Langley-Welding-Helmet-Filter-shades/dp/B00GAWFK02/ref=sr_1_1?ie=UTF8&qid=1422830159&sr=8-1&keywords=welding+glass
This example even has two reviews - both of which are by people saying they're experimenting for photography, coincidentally. At £1.50 (around $2) it's probably worth a go.
I know you mentioned that you're not looking at a filter holder for square filters, but for the price it is well worth consideration