I assume "superb condition" means that they are unmounted? I can't see the background too well on these photos. Is it horizontally lined or cross-hatched? Can you post a clearer photo with some more detail? The first issue of these stamps (1893, Barefoot #3) has an indistinct coat of arms behind the numeral, on a cross-hatched background. It is marginally more costly than the 1894 issue (Barefoot #6), which has the Republic's coat of arms clearly shown behind the numeral, with a horizontally lined background. It looks like you may have the first issue here.
As to your question, there's no real "rule of thumb" for calculating "value". This depends on what people are prepared to pay. An unmounted mint multiple definitely commands a good premium over smaller multiples or singles. On this basis alone, please don't split the block!
A dealer will probably pay you between 25 and 40% of what they want to sell it for, because the margin is what they make their living on. Some dealers have a higher margin than others.
If you really do want an honest opinion, contact Alan MacGregor directly through the http://rhodesia.co.za
site and ask him the questions. I've dealt with him for years, and he will give you an honest opinion. He's based in Cape Town and you never know, he may want to buy it from you if it's good enough