Burping Drains & Other Assorted Mayhem

Greetings All, In my master bathroom, both (side by side) drains do not drain as they should and they both burp constantly when water is flowing. In this same bathroom, the toilet doesn't always complete its flush unless I hold the handle down completely until it finishes. I am making the assumption that because these issues are all in the same room, they are related. No other drains or toilets in the house exhibit this problem. From what research I've done online, it appears that my problem is vent-related, but I have no idea of how, where, when or why. I would assume this is something for a qualified plumber, but what kind of costs could I expect and is this something that will most likely re-occur unless changes are made to my system? Any and all info would be greatly appreciated, especially if I can resolve myself...
Thanks, Bob

drain

There is no way to estimate the costs because it will depend on how much time it takes to determine the problem, what is involved to fix it, and the fees of plumbers in your area. If you have a mechanical vent for that bathroom, it can cause those symptoms if there is a stoppage further down the line.

Clear Drains

I would be more inclined to look to your drains than to venting. Has it worked previously without problems. Did they all three start not draining at the same time? Is this bathroom the closest to the main? What two drains burp? If you're saying the lavatories then do they have seperate traps or are they tied in together? How does the tub drain?