screening in a porch

Impossible to say how much without a few measurements. You're going to have to frame in the screening between any posts holding up the roof using 1x2 or, better, 1x3 lumber and have some kind of 2x4 base screwed or nailed into the porch itself. Do a little google on how to make screen frames, it's pretty easy. Screen costs between $1.00 to 2.00 a running foot depending on how wide it is and whether its metal wire or fiberglass. Fiberglass has improved in recent year so don't listen to the naysayers.

Hey look at this: http://www.diynet.com/diy/so_patio_rooftop_gardening/article/0,2029,DIY_14358_2270668,00.html

Just keep picturing yourself sitting on the screened porch at the end of a summer's day, looking out at the world and beyond.

L.R.R. - be sure to confirm that in your jurisdiction you'd be able to do this without a building permit.

Very good point.

just have to share a little joke with you before you proceed;

Remember the "Wizard of Id"?

well the wiz' wife was complaining that she had nowhere to sit outside in the fresh air, so he built her a deck.
Soon she complained that there was no protection from the sun when she sat on the deck, so he built a roof over it for her.
Then she complained that when the grandchildren came to visit, she was affraid they might fall off the deck, and hurt themselves, so he built a railing around the deck.
Next she complained that at night the mosquitoes were annoying when she was chilling on the deck, so he screened in the deck for her.
Then she complained that in the winter the deck was too cold to enjoy, so he added windows, and heating to winterize it.
Now she complains because she has 'another' room to keep clean!

Thank you all for the advice... and for the joke, that was great!

the joke had a serious side; plan exactly what you might do - long term - before you begin any work!
If you might decide to add windows later, make the screens removeable, for example.
"Plan first, build later"!
[and, by the way, in Toronto, Canada, no permit is required for non structural changes.]