Diano, I just completed my divemaster course so I may be of some help. For the past three months I have been diving and studying, and occasionally partying, in Utila, one of the three bay islands off the coast of Honduras. Utila rivals Thailand in the cost of courses and both places are known as the cheapest places in the world to dive. There are about 14 shops on the small island so prices vary somewhat. The average divemaster course costs about $500 US, some alittle more some alittle less. Im not familiar with BSAC certification levels but if your current certification is equivalent to PADIs open water then I dont think you would have to take a cross over. If anything a dive shop might suggest you take a scuba tune up which is basically a review of the dive skills PADI requires for successful completion of their OW course. If you can show that you are able to perform these skills without difficulty I would not think that the dive shop would have any problem taking you on as a DMT (divemaster in training). In PADIs world before you can progress into your DM course a few other courses are required. These course are the advanced diver, EFR (which is basic first aid and is does not require any water skills), and the rescue diver course. Average costs for these courses on Utila are $150, $60, $250, receptively Most of the schools will keep you on as a divemaster but be warned pay is little more than a nice dinner and a beer. Back to Utila. Utila is a backpackers paradise where tons of young travels mostly between the ages of 20-30 who are transacting Central America make a short stop over to find out what the SCUBA buzz is all about. Do not go there expecting posh hotels which wait on you hand and foot. Rooms cost about $5 a night, some as cheap as $3, and for the most part are nothing but four white walls, a bed and a fan. If you are staying for an extend period there are apts you can rent ($100-200), again these are basic dwelling. Everything is pretty basic ont he island. If you want to live more extravagant check out Utilas sister island of Roatan. Pace on the island is slooooooooowwww, but hey what do you expect its the Caribbean. All in all the best part of the island is the people. The locals can come off a bit harsh at first but if you make friends with a few then they are wonderful people with great stories of life on a small Caribbean island that in most peoples terms is far behind the rest of the world (the island just got 24 hour power in the last year). The travelers have their share of stories too and bring them from all corners of the globe, I spent many a sun set at the local bar staring out to sea with some total stranger hearing about some place I had never heard of. If you would like more info on the island, places to stay, impressions of diveshops, whatever, let me know. I would be glad to help.
oh and one more thing, the level of education in a divemaster course varies greatly. Just remember you get out what you put in.