We got our silver several weeks ago at about 3 1/2 inches and he's now 10 inches and doing great.
20 dollars for 4 inches seems to be about normal right now, perhaps a little bit cheaper than normal.
I don't think ours was any harder than any of our other fish. We simply made sure to acclimate him well and put him in a tank all by himself with a tight fitting lid. Barebottom 55g with black spraypainted back and bottom, filter and air wand. It took him awhile to settle in but he started eating right off the bat.
Try not to overfeed, keep water quality good just like any other fish. Keep the lights off for the first couple of days to ease stress, and use subdued lighting or cover in general.
Ours did seem to get stressed or a bit nervous easier than any of my other fish, they can be kind of jumpy. You need to give them time to settle in, don't mess around with the tank too much during that time other than feeding and water changes. Try to keep water changes small until he's settled in and more used to surroundings. Try not to stomp around the tank or move extremely fast because they will startle and hit the glass. Ours eventually got used to us though and startles much less now, in fact he comes to the front wanting food whenver we're near.