The thing is, is that it isn't actually as cheap as 10 pesos per text, as for 200 pesos you'll get 20 texts, but the 200 load costs £10. If there are 70 pesos per pound, really if you're paying £10 you should get a 700 pesos load, so you can see how much money they'll make! So £10 for the 20 texts you'll get works out at 50p per text or around 35 pesos per text.
Edit!: That's with the sim card, with the loads it's:
P200 (doesn't say how long valid for) x3 -£10
P300 valid for 75 days -£4.80
P500 valid for 120 days -£7.50
P1000 valid for 120 days -£14