Alternatives to CalypsoBTS?

Hello GSM network side folks, I assume that most of you are probably aware of the existence of a certain hack called CalypsoBTS: it's a rather unbelievable hack that takes a piece of hw meant to be a GSM MS and turns it into a poor man's GSM BTS, all inherent asymmetry in the GSM air interface be damned. As a manufacturer of Calypso-based GSM MS devices I occasionally get approached by people who seek to acquire a Calypso device seemingly for the sole purpose of running that CalypsoBTS hack, and every time I get approached by someone of that sort, I always shake my head in bewilderment - isn't there a better way to get your own little toy BTS running than to misuse GSM MS hardware? The purpose of the present post is to solicit advice from the community as to what I should tell those poor souls who are seeking to set up their own toy BTS and are looking to do it via the CalypsoBTS route - is there some better way that we as a community can steer them toward instead? It is my understanding - please correct me if I'm wrong - that the least expensive way to set up your own GSM BTS for toy purposes (as opposed to running a real operational GSM network which people will depend on for real communication) is to use a generic SDR device of one of several types that are supported by osmo-bts-trx/osmo-trx - but this is where my knowledge in this area ends, as this particular mode of GSM toying has never been an area of interest for me. But for those people who (unlike me) *are* interested in setting up their own toy BTS in the least expensive way, what SDR hardware should we as the community recommend for them? What is the cheapest option that will be good enough - especially if the criteria for "good enough" are compared to CalypsoBTS? What would be the least expensive option that is just good enough to be advocated as a better alternative to the CalypsoBTS hack? In the case of my current FCDEV3B hardware the price tag seems to be an effective deterrent against people misusing it as CalypsoBTS: FCDEV3B is expensive, and someone whose actual need is to run their own BTS rather than MS can easily buy a suitable SDR device for the same price or less, it seems. But the issue is beginning to rear its ugly head again because I have another development board in the works (not here yet, probably won't have the hw until December), and there is a possibility (nothing is certain yet) that it might be a bit less expensive than FCDEV3B. Is there an SDR-based option (or any other non-CalypsoBTS option) for running a toy BTS with osmo-bts-trx and the Osmocom CNI stack behind it that would cost $250 or less in hardware? The $250 number comes from my anticipated-around-December new FreeCalypso development board - there is a chance that it might be that cheap (but again, absolutely nothing is certain at the present moment) - and if there is no SDR or other option for running your own toy BTS for the same or lower hw price, then I fear that I am going to be flooded with support requests from people asking for help with using my new board as CalypsoBTS, which I absolutely dread. Hence my present attempt to pre-emptively seek some better solution for those people. (It would be nice if that stream of support requests from people seeking to run the CalypsoBTS hack could be redirected to Sysmocom or some other commercial entity who could make some money helping those people, but my experience is that these people are not the kind who would ever pay for commercial support, so no hope there...) It may also be worth mentioning that the filter replacement hack (removing or replacing Rx SAW filters that are meant to limit the GSM MS device's Rx capability to only specific GSM downlink bands) will not be possible on the new FreeCalypso GSM MS board that will be coming around December - that design uses an integrated RF FEM (front end module) instead of discrete antenna switch and SAW filter components. But I've also heard that plenty of people run the CalypsoBTS hack without doing any filter rework, just letting the strong signal from a nearby GSM MS force its way through wrong SAW filters and not caring about the 40 dB or so of attenuation being incurred - I cringe at the thought, but that's what people do... M~