Yes, I have saved tons of money by growing my own yeast. It just takes a little planning and time. Slants or glycol storage are going to be your best bet. Get a pressure cooker to acts as a makeshift auto-clave for sterilization. With some yeasts coming in at $6-$9 a vial, this will help you get the most out of that money. In fact I have pulled proprietary yeast and grew it in my house. I dont know the price you can put on that, but hey i have special yeast now. Initial investment will be roughly: 50ml Flask, 500ml Flask, 2000ml Flask, inoculation loop, agar, DME, alcohol lamp or just your stove, auto-claveable slants or pre-made malt-agar slants. So with a little investment, you can start saving money on your yeast. Look at question 3.
Always work near an open flame! You will want that updraft to keep any bugs out of your harvest. It also gives you easy access to sterilize equipment. If you are going to pull yeast from a starter: Take out a slant from a star-san bath and carefully remove the lid near the flame. Place the cap back in the star-san. Next hold the lip of the slant to the flame to sterilize, but dont melt the malt-agar. Take a inoculation loop, and hold it in the flame until red. Pull the loop away and allow it to cool for a few seconds. Stick the inoculation loop into your starter and swirl the loop around the starter. (Again still working by the flame) Take the loop out of the starter, (you dont need to see anything on the loop, there is yeast on there) and rub the loop in a zig-zag motion on the malt-agar inside. Stay away from the sides of the tube, try to stay mostly in the middle. Remove the cap from star-san and screw on tightly. If paranoid, wrap electrical tape around the cap. Place in a cool dark place for 3-4 days. Check on the slants for mold and yeast after the 3-4 days. Throw away any infected cultures, just not worth it. The goal will be to see 2-5 separate little white dots. These will be single yeast cell colonies, which is awesome for QC on your beer. If there are streaks, or connecting colonies that is fine too. That just means they are multi-cell colonies, and will just increase the chance for yeast flavor change over generations. If there is yeast growing, open the lid near flame to "burp" the slant. (yes the yeast will still give off co2). Reseal and let sit for a few more days, and then put the slants into your fridge.
When you are ready to get a starter going for a brew day, you will need to start about a week or so ahead of time. Boil up some DME and put about 25ml of it into a sterilized 50ml flask. (remember work by flame) Take your selected yeast slant, give it a dunk in star-san, and open the cap near the flame. Hold the lip to the flame just to kill anything at the end. Sterilize your inoculation loop, and let it cool. Swipe the loop over ONE of the single cell colonies, not all, and swirl the loop in the 25ml of wort. Cover and place in a cool place, swirling every once in a while for 2 days. If its a multi-cell colony, just get a good swipe. After the 2 days you should start to see some yeast at the bottom of the flask. Prepare the 500ml flask with 150ml or fresh wort, and dump the 25ml flask into the 500ml flask. Again store for 2-3 days, swirling or putting the flask on a stir plate. Optional but recommended for a good healthy yeast, add some fresh wort to the 500ml flask after the few days bringing it up to around 400ml. Sit for 2 more days. Now its time to make your starter. I will make a 1.4L starter, and dump the contents of the 400ml into the 2L flask. Set it on a stir plate, and let it go until the brew day.
This brings me to the last question. If you have multi-cell colonies, you can re-harvest 4-5 times getting roughly the same result, but it will mutate into a house yeast. Which is fine, and you can keep using that for more generations, but it will no longer be WLP500. Similar yes, but not the same as the generations move on.
However a single-cell colony will last for a long time. Since you are pulling yeast from a single mother cell, you have the possibility of less mutation. Say you have a 3 single cell slant. You pull colony #1 from Slant #1 the first beer you make. Do another slant of colony #1, if its bad, and you know it was the yeast and not another factor. Destroy the slant. If its good, keep the slant. Do this with the other 2 colonies in Slant #1. So if you can re-use yeast 3-5 times, and you have 3-5 single cell colonies each time you pull from a single cell.... Well that is a lot of math, but as long as you treat it right. You wont be buying yeast of that strain anymore. I try to maintain a neutral yeast, high gravity, a belgian, a english style, and what ever bottles I may want. Too many strains and it can get bad. So start slow.
You can also pull yeast from your beers after fermentation, from your carboy, but that requires washing the yeast, and a few more steps that I just dont think are worth it. So I always just pull from the starter going into the beer, and make notes on the yeast after that beer is done.
Hope that helps, hit me up if you have any questions.