Howard - Regarding evaluating mahogany for carving, most lumber dealers (dedicated lumber dealers, not retail stores that sell surfaced wood) will allow you take a small, shallow gouge to the surface of a board near the end to evaluate the color, grain, etc... In fact, there's a whole class of collector's tools called "timber gouges" that were specifically designed for this purpose. If not, I'd find another dealer. On rough-sawn stock, removing a shallow chip to look at the wood underneath makes no difference to the end user, who will plane/joint the board flat and remove way more material than the gouge will.
Regarding Irion - they do indeed have a 150 bf minimum for a shipped order (used to be 200 bf). This actually benefits the wood buyer - it's not so much that Irion won't ship a small board. The problem is that almost all of their wood are standard lengths - 10', 12' and 14', which must be shipped by common carrier (i.e. not UPS or FedEx). Common carrier freight carries a high front end that drops off rapidly as the weight increases. By having a 150 b.f. minimum, Irion ensures that you're not paying 40% of the lumber costs in shipping.
In fact, most shipping companies charge two different rates based on the "spot quote" system. Anything less than about 3000 lbs. will have a much higher rate per pound than the spot quote price (over 3000 lbs.), so much so that you can actually pay more in absolute terms for a 2800 lb shipment than you will for a 4000 lb. shipment.
What I'd say about the cash outlay is that one has to carefully consider their storage options to determine whether it's worth it, but wood, especially the kind of wood that Irion sells, does not depreciate over time. And if you plan on making period American reproductions out of south american mahogany anytime in the next 30 years, it might be worth taking out a bank loan to get what you want. Exportation of mahogany from south america is under increasing pressure from environmental organizations that see harvesting of this species as an incentive to road building and deforestation in the Amazon basin, and there are lawsuits pending or active in the courts designed to force the large American importers to drop mahogany from their list under the claims that they share responsibility for illegal logging from Peru.
Whether these lawsuits succeed or to the degree that they succeed may sensitively affect whether you can get any S.A. mahogany at all in the future.