The comparison isn't to Jose Miranda, necessarily. Payroll aside, the comparison is to the one year rental you can sign for 70% and no long term commitment. When that one year guy can be an RH outfielder/DH one year or a 1B/3B/DH guy another, something like Alonso is a luxury item.
Maybe you can be willing to get the premium one year rental at equal cost if its the perfect roster fit. I would be willing do an even comparison of production after 5 years and there's a pretty good chance Alonso comes in second.
Payroll considered, I have to think there will be a TV situation soon that allows something more than $4m and $6m for the rental spot. $15m can go a long way in this role, especially considering an Alonso basically squeezes cost controlled Miranda and Julien off the roster.