Mmmmm...
I haven't come across anything like that which is not linked to some other function. For example I know of online BOM management tools but they are for managing quotes and orders from manufacturers, not for your own inventory management.
MYOB/Quickbooks can manage BOM's and builds but obviously they care about the financial aspects. Most dedicated software solutions will focus on the change control / quality system aspects. And really the starting price there is $20K. There is this one at $1K but I don't think you are looking for that.
http://www.buyplm.com/
We do a lot of high value and complex projects and we still use Excel for the BOM's because everybody has it. Our suppliers can quote from it, we can export into financial packages and larger MRP systems, we can import it into online BOM builders at digikey/farnell etc.
Sorry.