I don't know if you can even find a 5 shot .45 long colt revolver...I've never seen one.
There is a 5 round .45 conversion cylinder made for some of the old cap and ball models. There a notch for the hammer so it doesn't rest on a live round. I am not 100% sure if this would be considered "legal" by any of the mounted shooting organizations. Unless you are really into old style pistols, you probably wouldn't have a rare configuration like this. I know several people with C&B pistols but none have the conversion kit.
Unless you already have the above, you are better just buying a .45 for mounted shooting.
New Rugers are about $600 now. Used around $500. An Uberti will cost a little less.
I was wondering how much are the close for this sport??
Clothes can be as expensive or inexpensive as you like. The "period" stuff can get costly quick. There are also companies which make patterns if you are handy with a sewing machine. I know a few folks who make their own clothes (mostly ladies). The do it yourself route can be very inexpensive.
Going with the "minimum" can also be inexpensive. CMSA and SASS both allow regular western shirts and modern 5-pocket jeans. If going this route, boots, a hat and chaps/chinks are required. To do away with the chaps/chinks requirement you need to wear "period" pants which you can find for around $60 new and less used.
If you are riding SASS, there is usually a little more leniency for new shooters at "local" matches. The rules say to give a new shooter about a year to get their entire "outfit" together. Check with your local club for their policy.
If you are riding MSA or CSA events, the dress code will be different. I don't have the MSA rules handy, but I _think_ you don't need chaps/chinks with jeans.
CSA allows clubs to set their own dress code, so check with your local club for details.
Remember that dress code usually only applies at sanctioned matches. You normally don't have to worry about it for clinics and practices.