When I added bikes to my Space Marine army, I went with the
"Dark Angels Ravenwing Battleforce". It is $105 (retail), and includes: Dark Angels Ravenwing Bikers (includes 6 Ravenwing Bikes), Dark Angels Ravenwing Attack Bike (includes 1 Ravenwing Attack Bike) and Dark Angels Ravenwing Land Speeder (includes 1 Ravenwing Land Speeder). In other words, 1 attack bike, 6 bikes and a Land Speeder. If you just want the attack bikes, not the best way to go, but if you want to add full bike squads, this is a great way to go. It also comes with LOTS of fun bitz, including dreadnought bitz.
I love the Land Speeders, but being open top with an armor of 10, they die quickly. Also, if you put 2 or 3 in a squadron, then immobilized = dead.
I do love my bikes...toughness 5 and twink linked bolters on a relentless platform unleashes massive firepower. They move faster than terminators and unleash similar firepower (the twin linked bolters are almost as good as the storm bolters on the termies, better in rapid fire range); they also cost less (25pts vs. 40pts--at least in the Space Marine Codex).
A squad of 8 bikes + an attack bike = 9 twin linked bolters and a heavy bolter (or melta). 9 bolters, rerolling misses means you have an 89% of hitting (8 hits), plus 2 more hits from the heavy bolter. At ranges of 12" or less (rapid fire range), you get two shots each from the bolters (regardless of move thanks to relentless) or 16 hits on average. Against T4, this means 8 wounds and 1 or 2 more from the heavy bolter on the attack bike. (Sorry, I have been practicing my "math-hammer" lately).