It is a bit more complicated than the two originals separates options: bunker (2 IPC for 1 additional hit) + reinforced position (1 IPC for a +1 def).
But it gives more value for IPCs.
And it allows 2 ways to attack this fortification (not only land, but also air) to increase the tactical options for the players.
A more simple way to deal with it, is to consider them as independant buying (for those extra-bucks “left over” after the main purchase):
For 1 IPC, you give to an Inf or Art +1 on def. Up to 3 units stationned in a given territory.
For 2 IPCs, you built a bunker that gives 1 extra hit to fend off an attack in a territory. Max 1 per territory.
If defenders survive but used the extra hit, it cost 1 IPC to repair the structure.
It can be SBR, but it is done differently: a single roll of 1-4 means it is destroyed, a roll of 5-6 it is undamaged.
If their is a AAA in the territory, it get 1 @1 shot on the bomber.
All these in a single territory will give: 3 units (Inf/Art) defending @3. And an extra spare hit. For the cost of 5 IPCs.
So a reinforced defensive position of 3 Inf and a bunker gives: 12+2= 14 IPCs/ 3@3 / takes 4 hits.
It’s seems superior to 2 Inf+ 2 Art= 14 IPCs 4@2 / 2 Inf+ 1 Arm= 14 IPCs/ 2@2 and 1@3 /takes 3 hits.
And reinforced defensive position of 3 Art and a bunker that gives: 15+2= 17 IPCs / 3@3 takes 4 hits,
seems slighlty inferior to 3 Inf and 2 Art: 9+8= 17 IPCs / 5@2 takes 5 hits.