Good post Zafra - and I agree. It is hard to compare the costs between printing at home vs printing by a lab (be it a pro lab or the likes of BigW/HN etc).
Considering you already have a rapport with a pro lab minutes away from you I'd be taking advantage of that if I could.
Perhaps they could offer slightly better pricing if you became a regular?
It is also worth noting that wherever you have your prints done, if you want them to look their best you will need to calibrate your PC monitor/settings so what you see on your screen resembles how the photos appear once printed by your chosen lab. This is one plus to a pro lab, who consistently keep their machines calibrated, where some labs like BigW etc can be lax at doing so, resulting in one print job being fine and the next having colours slightly off. Perhaps no biggie if you're printing a few 6x4's for friends but not so great if you're doing an enlargement you want to hang in your house (and look at every single day).
And I ALWAYS resize my images to the exact pixel dimension required for printing when saving to JPEG before taking them to the lab. I prefer to use Lightroom or Photoshop to achieve this instead of letting the machine at the lab do it for me - but I am a control freak!
To do so you simply need to use this formula:
required length of print (in inches) x DPI being printed = pixel dimension for length of image
required width of print (in inches) x DPI being printed = pixel dimension for width of image
So, a 6" x 4" image printed at 300dpi (the standard dpi setting used) would result in:
6 x 300 = 1800 pixels in length
4 x 300 = 1200 pixels in width
So when you resize your RAW image, you select 300DPI and then choose 1800 length and 1200 width and you have a perfectly sized image, ready for printing at 6"x4" size @ 300dpi.
If you end up printing a 30" x 20" at 200dpi the same formula applies.
I used to print images at home (a long time ago) but ink is so expensive and seems like it gets used faster than fuel in a car, so I never do my own prints at home anymore (unless it is a one-off thing that I need immediately for some reason - kids projects for school etc. The printer we have now we purchased because JB HiFi had it on special and it was cheaper to buy the whole printer (which came with ink) than it was to replace all the colours of my previous printer, which was the reason we'd gone to JB in the first place. Ridiculous how the entire printer could be cheaper than replacing just the ink tanks.
Dave