RamAir,
No problem getting larger prints from the slides. All the big airline type posters you see in travel agents and just about all good quality posters were taken with Transparencies (Slides). They are usually done in a larger format though (not 35mm). I have some really nice original Australian Airlines A300 slides, air to air shots on larger format that were
rescued and the quality is amazing.
The chemical process for Black and White is actually cheaper
BUT because there isn't a large demand from the general public, your local 1 hour developer will charge you more just for the trouble to send it to a lab. Supply and demand is what they call it
.
Not sure where you are but if you are in Melbourne, try VANBARS in Carlton, they
ARE very professional and good, i.e. you don't get prints developed with dust specks, half the nose or tail chopped off from the frame and the colours are just right. They have ALL the nick nacks you need for your habit and sell the cheapest Slide Film that I can find. They are sort of like a small warehouse.