I posed a similar question (or at least a question which raises similar issues) on here a few months ago: Moving tfrom Shimano 9- to Shimano 10- speed
The issue is not the different Shimano ranges per se, its going from 9 to 10 speed.
You can imagine that the 10-speed shifters will be expecting a 10-speed cassette at the other end of the wire - these things are indexed after all.
The other complication in going from a 9- to 10-speed cassette is the chain. You'd need a new, slimmer one. And unfortunately this means a new real derailleur too. Strictly speaking, this again knocks on to the front derailleur. And the chainrings. But as regards the front setup, I was advised that I should be able to get by with my existing setup.
Unless you specifically want to go to 10-speed, you may have some luck doing what I did - jump over 105 altogether and head for the old Ultegra 6500 range.....which was 9-speed. I suspect if you could find shifters from this range, it would do the trick. Alternatively you could go not quite as far and upgrade to Tiagra - I believe the current Tiagra range is now 10-speed, but its not so long ago that it was 9-speed. (The bike I raised my question about was a 2011 model.) Obviously Ultegra will give you better quality but you'll still find Tiagra shifters a noticeable step up from Soras.
If you really do want to go to a 10-speed 105 setup, well I think I costed it as something like GBP400 or more...not a lot less than buying the whole 105 groupset (which can be bought as a single product). But I seem to remember the biggest single cost was the shifters, so take that as an indication.