I joined so 49.99 for them

Probably very little, theres even the chance they may lose money on the service initially.

i hope they do, i think they will get a decent profit.

and if they do cheaper ps3s :D

What would they be losing money on?

well my friends list has around  20 people and 5 of them now have a PLUS sign next to there name.

is that a good thing or not so good?

But that isn't pure profit.  Revenue maybe, but profit-wise I'd say they are still in the red for PSN Plus given the costs to start it and the initial wave of arguably better software in the first month that was available for free.  Once it gets to later months with software less people want in the free offerings they will probably start to make a profit.

Well judging from what people said in the other thread and average of 5% of people have PSN

So 35,000,000

5% = 1.7 million

1,700,000 million * £40 = 68,000,000

In reality there are probably 2.5% of the total PS population as most of us live in the richer countries so they will have made at least £34 million if 2.5% of people have it.

What costs to start it? It had basically no external marketing...

And £40 easily covers all the costs of giving away the content for free and then more. For example with Wipeout HD they will have broken even on it ages ago, and it will cost them nothing to distribute and sell it.

Additionally, it's not like they didn't already have a set infrastructure in place.  I would wager pretty heavily that PSN Plus is already a profitable success, although I highly doubt it's a significant profit in terms of the grand scheme of things.  But hey, it gives them an additional source of revenue and also gives users several options.  It's the route Microsoft should go IMHO. 

only 1 person on my friend list has psn so im guessing not much, about 100,000 subscribers