Have you asked the customer why he went for the other guys? If so then ask why they think it can be done for that price and what contingency they have from your competitor if they are not happy with the results or if the area is damaged by failure to re-sand etc.
Remember most customers are not happy telling someone they did not feel they were the right people for the job and giving a reason such as: " We spoke to one your customers and they were not impressed with the results" or "We felt you were offering more than we needed." etc so they just say they had a cheaper quote.
Very rarely is price the real problem with quality customers unless you are way over the market rate. Dig deeper with your customer but be diplomatic and allow them to be frank with their reply. If price is the only criteria then move on unless you want to be working in bandit country for the rest of your days in business.
