starphilosophy :
How likely is it to damage the mother board
when a psu blows it's a coin flip what can happen. it can start a fire and burn your house down. or it can kill everything (and I mean everything) in your computer, or it can take a part or two with it... or it can do nothing other then die.
Its basically roullet. You never know what will happen, what other parts may be destroyed or what ancillary damage a blown psu will cause. You basically have to replace the psu and pray that nothing else was blown.
This is why buying a GOOD psu is so critical. It's literally the only part in your system that will kill other parts AND one of two parts that can start a fire when it fails. The better PSUs tend to have redundancies built into them to limit the chances of critical system damage when/if the unit dies (no psu will last forever), cheap ones rarely do. So when you ask how likely it is, i'd say it's inversely proportional to the original quality of the psu.
find yourself a new psui off this list, from tier I or tier II. anything lower then that ranges from unreliable to dangerous.
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html#xtor=EPR-8886