I have my doubts as to whether such a complex, complete, and flexible program could be arrived at through traditional low cost open source means. Those with the money to properly manage such development want a return in their investment resulting in the high prices you want to avoid; those without the deep pockets don't have the resources (time and/or money) to properly manage the development to get the clean, organized results.
Keep in mind the automation behind the scenes which deals with a lot of the "I" in BIM - that automation dictates the work flow that must be followed. There are always concious decisions to sacrifice potential work flow options in order to support the automation. If the freedom of work flow was allowed in certain areas then the automation would be either massive and bulky (if not outright uncodeable), leaving the end user with a bloated piece of software or manually managing the data.
I really think you may have the wrong tool for the jobs you are working on, and something much more lightweight would be appropriate.
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If you are going to fly by the seat of your pants, expect friction burns.
"I don't know" is the beginning of knowledge, not the end.