Improved, 3 Star, Uncommon runes benefit your hero about the same amount as adding 1 star to the hero. Greater, 4 Star, Legendary runes benefit your hero about the same amount as adding 5 stars to your hero. The table above shows what level runes you'd need to achieve the same effect as having more stars on your hero.
I thought it might be useful for people to know the impact runes have on heroes compared to the impact of a higher number of stars on a hero. For example, if you are facing a PvP opponent with all 6 star heroes on their team, but with relatively low roster power (indicating they don't have runes or at least not very good runes), and your team has all 3 star heroes, but good runes, you still might be well matched against the higher star heroes.
The table above uses a Level 65 Igorak, comparing him totally un-runed at various star levels to a 1 star Igorak with runes. I know that Igorak unlocks at the 3 star level, but I just calculated what his stats would be at the 1 star level for comparison purposes. So for example, in the table you see that a 2 star Igorak, with no runes, is comparable to a 1 star Igorak with Improved, 3 Star, Uncommon runes, so those runes are worth about 1 star difference. On the far right of the table, you can see that a 1 star Igorak with Greater, 4 Star, Legendary runes is comparable to a 6 star Igorak with no runes -- so a 5 star differential can be overcome with good runes. Of course, your opponents will normally also have runes, but still thought it might be interesting to measure the value of runes compared to stars.
Note that hero level differences are much less significant that star level differences. You only add about 1.4% to your stats for each hero level you progress. However, keep in mind that if you go against a higher level opponent, you will miss more often and they will resist your spells more often as well.