Bingo PPC is expensive and expect to pay close to £40 or thereabouts per a conversion using Google PPC and yahoo/msn are not much cheaper. You will need to invest a large amount to see good returns. Smaller networks carry a higher risk. If you have an investment in excess of £10,000 I would suggest PPC/ Otherwise I would explore other forms of marketing as you will be throwing your money away.
For example a £500 budget will be a waste.