What would you call inexpensive? Given two suitable torque convertor housings (one from an original box, one from the new box - you need the front end of the former and the back of the latter) these can be made for somewhere in the $100-$175 range. Count on a driveshaft job as well (box length is different), and some custom wiring, crossmember, perhaps a few extra mount holes in the flexplate, etc. There are at least two driving around in the Netherlands, plus a bunch of manuals with the later M46 or M47 boxes, and there's a guy making the bellhousings (or rather he did the engineering on it and has a professional welding company do the actual fabrication -- _very_ good quality welding).
I am waiting if he ever manages to do a limited production run of a prototype modified M46 / M47 top cover he had some time ago that would allow it to work with a long stick (early 140 and earlier models). Oh well, I'll see him next weekend and ask.

On my own site, www.buvic.com, I've put a description of doing a similar sort of trick with a manual box: M46 into my '74 164, again with a modified bellhousing.
As George already remarked, the B30 mating face is larger than the B18/B20 mating face, so you won't be able to use the very same adapter. The basic concept and cost is about the same though.

And one minor addendum point to another remark of George: The M41 is for the B18/B20 engined cars, the M410 is mainly for the B30 engined cars and a limited number of B20 engined P1800 (with a different bellhousing than the B30). To the best of my knowledge there is no original bellhousing for M41 to B30.

Bram