So after going over the unchained summoner, I decided to see if I could figure out approximate guidelines by finding common threads to help people out with making subtypes for outsiders (and non-outsiders like dragons or Fey) eidolons. When possible, I'm going to try and give an approximate measure of evolution point value for each level, as that's the closest unit of measure one can use in this context.
My Analys of each level's bonus:
Base Form Evolutions (5 ep):
This was easy as dirt, each form's inherent evolution for their subtype are always built from 5 points each (with one exception) with Bipeds and Quadrupeds always having two limb evolutions (Legs plus either Arms or another set of Legs) while Serpentine having more variation due to having less essential evolutions (Tail plus Tail Slap). Azata have less than normal, but as will be discussed in the next section, this is compensated elsewhere.
Level 1 Bonus (4 Ep):
This level for the most part was pretty simple, primarily thanks to the over abundance of resistance(Energy type) evolutions. The exact number for a good baseline comes from the azata, who only have evolutions from their basetype bonuses, which when you remove the extra point they have due to having less points in their base from evolutions ends up at 4 ep. Proteans also only have evolutions, with 3 ep in value, but their grab evolution is doubled up allowing them to have an evolution more than once, and can be assigned as they wish. The one exception to this rule is the inevitable, which in addition to not having abilities normally shared with other subtypes, also has an additional weakness in the fact they are susceptible to Bane/Favored Enemy/ similar effects that target constructs in addition to what they would normally be affected by, so I suspect that they have more than they would without that fact.
Level 4 Bonus (2 Ep):
wow, this level is just about the same for everyone here, either gaining resist 10 for two energy types, or an extra EP to build with. The only reason I was able to figure this levels approximate Evolution point value is that the extra Evolution point to spend also showed up in the Level 8 Bonuses (So a Free evolution point is approximately equivalent to 2 spent on pre-set evolutions). The exception, again, is the inevitable, who is off doing their own thing again, making their abilities hard to qualify
Level 8 Bonus (~2-3 EP):
Here is where the Evolution values start to get fuzzy, most things are straight 2 point evolutions such as winged flight, Constrict, or ability increases. Daemons get a bonus evolution point. The fuzziness occurs as first, Thematic non-evolution abilities start to show up, like Agathion's gaining lay on hands, then some abilities overwrite others like poison immunity replacing +4 vs poison saves. But the biggest outliers are Elementals, who end up somehow with around 4 points of evolutions like Flight(Magic) for air, +20 speed for fire (a little less valuable than 4pts of legs as it doesn't give limbs for extra natural attacks) and 3 points for burrow, or swim x2 and gills.
level 12 bonus (~2-7 ep):
Okay, this level has a similar issue to Level 4 in that, other than the four elementals, EVERYTHING GETS DR. Either nulled by an opposing alignment or Adamantine in the case of Psychopomps. The other component is this is really where subtype abilities that you see in the Monster's Splats are coming online, like all Good eidolons gaining truespeech and immunity to petrification, or psychopomps spirit sense. Really the only subtypes that gain evolutions at this point, in addition to DR, are demons who get a paltry 2 point ability increase evolution and PRoteans who get a 7pts in Flight(Magic) plus Blindsense.
Level 16 and 20 (Scouter Broke):
This is where it gets pretty much impossible to say the official "Weight" of the bonuses in Evolution points, as the only Azata gain evolutions only at 16 and that's pretty low compared to what the others are getting (Funny enough, 16 is the level that all evil outsiders upgrade at least one resistance to an immunity and gain telepathy) and of course the 20th is the usual over the top capstone abilities, like Teleport (At-will Self only) SLAs and the like. So what I can gather is 16th is for iconic subtype abilities that won't completely break open the game and 20th is for iconic subtype abilities that will break open the game, but will likely never come up in a normal campaign