Yes, it is an unethical practice to accept expensive or even inexpensive gifts from clients during the bidding process. In fact, it is unethical to receive gifts from clients at any stage of the job, not only the bidding stage. Bidding and other business decisions are made by human beings and hence are open to bias and interpretation. Ideally these decisions should be based solely on the merits of the bid, yet subconsciously the deciding person or team may be biased slightly by the person they are receiving favors from. And hence the bidding decisions would no longer be objective. As a rule of thumb, politely refuse all gifts, especially ones you can't afford yourself. In general, the value of the gift does not matter, what matters is whether the person can stay impartial to the client from whom gifts are being received. Usually, organizations have rules regarding gifts and those should be strictly adhered to, especially when they involve someone who can be benefitted by your decisions.