The lack of a trap under the shower would certainly be a cause of a sewer type smell as there would be little if any water trap to prevent any smells coming back up the shower waste pipe work.
Regarding the costs to install one – well I can only agree with our previous expert – get some more quotes for the job – it is impossible to price the work without seeing what is required to achieve the desired result.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians