The SSDs are so expensive, can I buy one with a small SSD drive and put in a 1TB 'mechanical' drive?
Nope. The redesigned MacBook Pros are too thin to accommodate disk drives. (Unless you opt for the
older, non-Retina model.)
In addition to being too thin, they no longer use the same interface. Spinning-rust drives communicate using a SATA connector. Apple's newest Retina Macbook Pros now talk to the SSD using a PCIe connector, which is far faster, but not yet standard. (Previous generations of SSDs have used the slower SATA connector because the older SSDs were not yet fast enough that the SATA connector was an issue, and because it made it easy to simply plug them in place of a spinning-rust drive. The new generations of SSDs are very fast, so Apple chose to connect them through the fastest interface they could, but it's still hard to find PCIe SSDs on the open market. You can do it, but they're eyewateringly expensive.)
Does that mean that it will now be very hard to find a SSD to trick-out my 13" (2011?) MBP? I saw a YouTube video of someone doing it.
No. It seems to me that tacit's comments refer to the current MacBook Pro models with PCIe interface. Your late 2011 model (MacBookPro8,1, MD313LL/A) has the SATA 3 interface for which less expensive (but slower) SSDs are still amply available. Technically
the upgrade is listed as moderately difficult by iFixIt.
that macfixit article shows replacing the optical drive with an ssd.
one thing to watch out for - bus speed. Way, way back, I pulled the ODD out of a G3 and replaced it with a hard drive. I was upset to find it accessed at around half the speed of the boot drive or other drives in the computer. Switching drives didn't help, the upper bay remained a problem. Come to find out, that interface to up there was ATA33, and the others were all ATA66.
So you may want to actually move your HDD into the optical drive carrier, and put the SSD in where the hard drive goes. YMMV, report back results, I haven't tried it recently. In this case the ODD is sata, but it may be 1.5 vs 3.0 for the hdd, as an example.