Based on napkin math from a long discussion about this issue with some friends in related fields, we estimated that a 0.5-1deg spot digital meter with a dial scale on the side could be made in something like the $30-50 retail range using mainly off the shelf bits... If we ordered about 100,000 of them in the first batch. - A bit more for things like flash metering support, memory/averaging modes.
That is a LOT of up front capital to spend on a fairly modest return, and given how many cameras these days include robust metering systems it would take an impressive advertising campaign to convince that many customers that they not only needed a stand alone spot meter, but that ours was the one they should buy over a used model [or pay the premium for existing and known 'professional' gear]
By the time we drop the up front investment costs to something more 'reasonable', our prices shot up into the realm of existing known brands, and we're then still out a good chunk of cash and still facing that uphill battle of convincing anyone to buy our new and unknown thing rather than an old tried and true.
At some point we might get bored and design it as an open source hardware project, just to keep something 'alive and available' in the community, but even that would take a lot of effort to do well.