It's actually not too bad, even on phones without "easily" replaceable batteries.
Speaking for the OnePlus One and Nexus 5 the procedure is pretty straight forward: pop it open using plastic trim tools (which are often sold with batteries) disconnect the ribbon cable and pry the battery off any adhesive. Drop the new one in, reconnect it and snap it back together and you're done.
Depends on the phone I guess, but the whole thing takes maybe 15 minutes.
eBay is your friend, you can get batteries for ancient phones like the Samsung Intercept for $4 still, brand new. Luckily for us, batteries are a highly commodified component, you can replace the cells yourself with minimal effort if you so choose.
When I'm feeling like an uber-cheapskate I rebuild laptop batteries, recycling the failed cells and replacing them with good cells from other batteries.
Speaking for the OnePlus One and Nexus 5 the procedure is pretty straight forward: pop it open using plastic trim tools (which are often sold with batteries) disconnect the ribbon cable and pry the battery off any adhesive. Drop the new one in, reconnect it and snap it back together and you're done.
Depends on the phone I guess, but the whole thing takes maybe 15 minutes.