The DMG states on p288 under Adding New Abilities:

A creator can add new magical abilities to a magic item with no restrictions. The cost to do this is the same as if the item was not magical. Thus, a +1 longsword can be made into a +2 vorpal longsword, with the cost to create it being equal to that of a +2 vorpal sword minus the cost of a +1 sword. If the item is one that occupies a specific place on a character’s body (see Magic Items on the Body, page 214), the cost of adding any additional ability to that item increases by 50%. For example, if a character adds the power to confer invisibility to her ring of protection +2, the cost of adding this ability is the same as for creating a ring of invisibility multiplied by 1.5."

So you can enchant your spellbook as you like (unless the spell description specifically restricts it). Other items if they take up a body slot incur the 1.5x extra cost. It goes without saying that the DM has the final say on what can and can't be done in their game world.

10g per item.

That's exactly how it works: you find and hire someone who can cast identify and they use their spellcraft skill to determine the properties of the item. (The charge is for the service performed – obtaining the benefit of a spellcasting. There's no service performed if they just cast it on themself and then sit there refusing to identify your stuff.)

Note that:

• one attempt at identifying an item using spellcraft takes 3 rounds
• the duration of identify is 3 rounds per caster level
• the cost of a spellcasting is caster level × spell level × 10 gp

So no matter how high level your NPC spellcaster is, it costs 10gp per item identified, assuming they don't fail the check (which they likely won't, with a +10 bonus due to the spell). Also, unless they're of high enough level to be able to cast multiple identify in one day and identify multiple items per casting, it may take some days to get a whole haul of items properly identified.

The trick then is in finding someone who can perform this service. Not every shopkeeper – even a shopkeeper of a "magic item shop", if you have those in your world – will be a spellcaster.