Sounds good. I am in favor of open source be a requirement for all software acquisitions by the public sector. More companies should follow that.
Now I understand that it may not be feasible to do an immediate switch in all cases. So to take that into account you can instead of a hard rule make the rule that any call for tenders must apply a 3x price penalty when comparing offers which include any closed source software components.
My reasoning behind that is that to me 2x the initial price seems like a sensible estimate on the future cost of eliminating your dependency on the closed source software once the contract ends or that software is retired.