If i remember right, i think you should be paying about 12thousand pesos (12 lucas) for the taxi, which is about 12 pounds.
As for the city i think you should visit the cerro san cristobal, the big hill in the centre, you can get a cable car that goes to the top from near the pedro de valdivia metro stop on the red line. Then you could walk down or get the fenicular railway into bella vista and see pablo neruda's house, if you're into poetry and all that. Bella vista has some good bars and is nice to walk round, as is bellas artes which is just over the river from bellavista. If you're not up for going to the top of cerro san cristobal, there is also cerro santa lucia which is mouch smaller but still offers good views of the city, its next to the santa lucia metro stop on the red line.
If you fancy going off track, i think recoleta is an interesting barrio, old buildings, poor but nowway near the worst in santiago. In recoleta you can find the cemmentary, there's two actually and the old catholic one is very nice and worth a visit if you have time. Thats on the yellow line, metro stop Cementerios.
You could also go to see the presidecial palace, la moneda, red line, metro stop la moneda. This is the one pinochet bombed before taking power, now restored you can pop inside in the day and there is a cultural centre below. I recently wen there to see an exhibition of rivera and his wife.
Still with time you could pop up to mercardo central at the stop Puente Cal y Canto on the yellow line. Here you can get some seafood dishes (chile's speciality) or you could pop over the river, two seconds, to the dodgier market called ''La Vega'' where you can see some interesting sights, poverty.
Also there is the main square Plaza de Armas, where the cathedral is, its ok, not brilliant, but all the toursits go.