Def not lasers
A planetary attack is going to be either out and out distruction or aimed at occupation.
I cant think of a defence against out and out destruction, such as attack could be any thing from bombing usung any type of material (boi, concussive, heat…. anything). But what would be the point. Such destruction wouldn’t be profitable as the huge cost would need to be re-couped somehow.
If the attack were occupational then the attacker would need troup carries and all the transport paraphinalia and that means slower less manouverable ships. Bettwe still, a defencive force to attack when the ship is landing and more vulnerable would be even better.
Personally, I would say the cheapo defence would be explosives used in a range of ways: Tagetting the large ships as they slowed down to enter the planets atmosphere, when they landed. Energy weapons could be deflected.
Also, the cheapo defence would engage in using the cheapest stuff they could, why blow a ship up with a huge bomb in space if you can just dissable it and let it drift.
To be honest though, I’ve never really thought of such a thing before. Interesting topic!