There are two sides to a transfer. The player's ability and the team's needs.
The player: 36 goals in his last 63 games. Apparently, since 2005 has been outscored by only Drogba and Roooney in the Premier League, whose last transfers were worth about 25m pounds each.
The team: Villa are outside relegation zone on goal difference. Relegation can cost a team about 25 to 30 million pounds per season. Only 4 teams have scored fewer goals than Villa, and no Villa strikers appear in top 20. (their top scorer has 5 goals). Given their lack of success and rotten location, they are unlikely to attract Eto'o and Pato.
Given the player's return, and the team's predicament, I find 18 million a high but justifiable price.