In the end, the value of DR should be the cost normal armor, plus the cost of negating any penalties that the armor normally applies. The normal penalties of armor are:
- Encumbrance (known way of negating: Payload). That works out to about 5 points per multiple of BL (cost of external payload)
- Concealment or social penalties (there is no particular advantage that lets you conceal worn armor, but the social penalties can certainly be bought off). In general lighter weight armor is easier to conceal and also has less of a social penalty, something like 1 point per 5 lb, or 4 per multiple of BL, or similar, seems fair.
- Armor layering penalties (flexible armor)/inability to wear armor (rigid armor). There is no particular trait that allows doing this, though armor mastery (DFA 28) includes it.
Overall, I would combine these as follows:
Natural Armor (5/level): your skin functions as armor. Pick an armor (or combination of armors) with a weight that does not exceed (level*BL/2) and a cost that does not exceed (level*base starting wealth/2), and your skin emulates that armor. The GM might require Cannot Wear Armor in the case of a rigid armor, but it's not required; a robot with a hard shell might still be able to wear armor on top of it.
- Cannot Wear Armor (-40%): you cannot wear armor over your natural armor. It is usually also Obvious.
- Limited Armor (-10%): you can wear armor over your natural armor, but are subject to the rules for armor layering.
- Limited Coverage (-0%): you don't need to buy a full suit. Pay for only the armor pieces you actually have.
- Obvious (var): You can't surprise people with the fact you're wearing armor. This has a basic value of [-1], but you might also take Unnatural Features, reduced Appearance, or other similar disadvantages.
At low TLs, one level probably won't get you a single point of DR over your entire body, unless very strong. By TL 12, a single level will get your DR 30 energy cloth.