If a company wants to lure a software engineer away from their current company they need to provide enough of an incentive for them to leave their current work place. I would wager most people probably wouldn't change their job (assuming they are content) for anything less than a 20% bump (all else being equal) but I would say people probably would want a higher bump than that.

20%: @ 100k --> 120k. @ 200k --> 240k. @ 300k --> 360k. 30%: @ 100k --> 130k. @ 200k --> 260k. @ 300k --> 390k