.....don't do it!
If your prime reason for blogging is to do SEO and so on, rather than because you've got something to say, it's probably not a good idea to do the blog in the first place: you'll quickly run out of things to say and so on. Besides, you'll see the blog as a chore (as your OP has implied).
In terms of whether it's value for money (WRT your time) you should be able to work that out pretty easily if you really want to, by costing your time and keeping track of how many leads you get per hour of blogging. For us it actually cut down slightly on the number of 'raw' enquiries we got but increased the value of each one we DID get because they only contacted us if they liked our style and wanted some presentation skills training the way we do it. Saved them and us time!