Hello David,
Welcome to the forum. I think the starting point is to get a prototype made - so that you can touch and feel the part - and may be even put your product in there, etc. I think you are in the UK and so do a web search on prototype and see who is there locally who can make you one these. You will have to give them a CAD drawing. Once you have finalized this, you can analyze the model to see what size machine you need, approximate cycle times, approximate mold costs and so on. Once you have this, you can then see if it is cost effective for you. You will not know the cost without doing these. Someone can approximate but there are still too many factors.
Hope this helps.
Suhas