Getting 50 minipets is very easy, if you buy from trading and auction websites. If you have the money. Don't bother with ingame looking for minipets - it's too tedious, since your minipet collection must contain only distinct minipets - no duplicates allowed.
Make yourself a list of all affordable minipets, start with
http://wiki.guildwars.com/wiki/Minipet. Exclude all rare, event or asia minipets. Focus on the yearly birthday minipets, excluding the green ones. As green minipet, create yourself the Black Moa Chick. As gold minipet, buy some cheap 1st or 2nd year gold. Check all you already got.
Focus on these:
There are 8 white minipets for 5 years = 40 minpets
There are 3 purple minipets for 5 years = 15 minipets
+ 1 cheap gold (1st/2nd year) + 1 cheap green (Black Moa Chick)
I just finished buying approx. 35 for my secondary account (I already had 15). It took about a week.
About prices: first I browsed the websites and noted offers in my list. This gave me an overview over the price levels. Then I began to buy from the cheaper offers and ignored the expensive ones.
From my observation, the prices were currently (last week of Oct 2011) like this, more or less:
| year | color | count | price |
| 1 | white | 8 | 1-2k |
| 1 | purple | 3 | 8-10k |
| 1 | gold | 2 | 30-40k |
| 2 | white | 8 | 2-5k |
| 2 | purple | 3 | 23k |
| 2 | gold | 2 | 30-40k |
| 3 | white | 8 | 8-10k |
| 3 | purple | 3 | 25k |
| 3 | gold | 2 | don't buy |
| 4 | white | 8 | 18-22k |
| 4 | purple | 3 | 30-35k |
| 4 | gold | 2 | don't buy |
| 5 | white | 8 | 25-33k |
| 5 | purple | 3 | 30-70k |
| 5 | gold | 2 | don't buy |
It's only an indication, of course. Prices change every hour, and this list is probably already outdated. I bought from german communities. US communities may have other price levels.
Building your hom-points with minipets is dirt cheap, if you compare it with the consumable titles.
If you buy all 50 minipets, the total prices may be like 92k (1st year)+101k (2nd year)+140k (3rd year)+250k (4th year)+200k(5th year)=783k for 8 of 50 hom-points, which is much much much cheaper than any of the consumable titles (buy all 10000 points with 100g per point=1000k).