There's a weekend sale on Comixology (app or website) discounting the Costa 2009 series to $1 an issue. The sale is on until the 10 June (US time)

All 31 issues are included individually or in the collections. Collections are a little bit more expensive on the app than buying separately. The fourth collection contains the Heart of Darkness miniseries which isn't on sale as separate issues.

If you started reading with the current MTMTE ongoing I'd suggest picking up issues 22 & 23 which James Roberts wrote. They are a pretty self-contained flashback-heavy story that leads into the flashbacks in the Shadowplay 3-parter and introduces many elements of pre-war Cybertron that Roberts is still fleshing out.