The following screenshot shows the ending scoreboard for a solo-queue'd game that I played, against what I'm fairly sure was one premade team (purple), and less-surely another one (red). Purple's healer made an enormous difference in his team's performance, yet his score was tied for 9th place, out of 12 players. If someone is putting out that close to 1 million healing for his team, he most certainly shouldn't be scoring so low.
'Course, this is just one screenshot from one game; there are plenty of games where I've personally had very high healing numbers, but not gotten very many points for it, and there are some other things that should probably award points as well (ie, being nearby when a teammate returns your relic, or defending your own team's relic, etc...)