There are a couple of problems with the premise of your scenario- namely, that you would be in a position to sell the head of a recently deceased holy man.

Typically, if you were walking around with an alleged relic, the likelihood that you would be able to pass off said holy person's head is certainly on the low end. Even if you had managed to acquire the cranium of a recently deceased (and likely popular) saint, I am not sure you would be in the position to name your price, and you might likely be subjected to a couple of questions in response.

The fact is the medieval church was highly suspicious of individuals who turned up with relics for sale or veneration, particularly ones that no one seemed to know much about. It's a little earlier than your 1300 time frame, but ecclesiastics like Guibert of Nogent were horrified by how susceptible ordinary folk were to potentially fake relics and those that marketed them. Consider Guibert's recollection of an incident involving the veneration of a recently deceased child:

I have indeed seen, and blush to relate, how a common boy, nearly related to a certain most renowned abbot, and squire (it was said) to some knight, died in a village hard by Beauvais .on Good Friday, two days before Easter. Then, for the sake of that sacred day whereon he had died, men began to impute a gratuitous sanctity to the dead boy. When this had been rumoured among the country-folk, all agape for something new, then forthwith oblations and waxen tapers were brought to his tomb by the villagers of all that country round. What need of more words? A monument was built over him, the pot was hedged in with a stone building, and from the very confines of Brittany there came great companies of country-folk, though without admixture of the higher sort. That most wise abbot with his religious monks, seeing this, and being enticed by the multitude of gifts that were brought, suffered the fabrication of false miracles. Even though the covetous hearts of the vulgar herd may be impressed by feigned deafness, affected madness, fingers purposely cramped-into the palm, and soles twisted up under men's thighs, what then does the modest and wise man, who professes to aim at holiness, when he makes himself the abettor of such things? Oftentimes we see these things made trite by vulgar gossip, and by the ridiculous carrying round of sacred shrines for the sake of collecting alms; and daily we see the very depths of some man's purse emptied by the lies of those men whom St Jerome calls rabulas in mockery of their rabid eloquence; who shake us so with their rogueries, and bear us along with such religious flattery that (to quote the saintly Doctor again) they gobble more busily than parasites, gluttons, or dogs, and surpass ravens or magpie with their importunate chatter.

And another incident:

This man, after a long and exaggerated discourse on his relics, brought forth a little reliquary and said, in my presence, "Know that there within this little vessel some of that very bread Lord pressed with His own teeth; and, if you believe not, here is this great man" - this he said of me - "here is this great man to whose renown in learning ye may bear witness, and who will rise from his place, if need be, to corroborate my words." I confess that I blushed for shame to hear this; and, but for my reverence of those persons who seemed to be his patrons, which compelled me to act after their wishes rather than his, I should have discovered the forger.

All this to say, any average Joe wheeling and dealing relics would immediately have fallen under suspicion of forgery. If you've found yourself in possession of a relic, best not to go try selling it unless you're in the position to prove the quality of your relic. You can read an excerpt of Guibert's much longer treatise On Relics here. Guibert also recalls a scenario in which Odo, the bishop of Bayeux, is duped out of one hundred pounds in his purchase of the relics of St. Exuperius, concluding:

I can recall so many like deeds in all parts that I lack time and strength to tell them here; for fraudulent bargains are made, not so much in whole bodies as in limbs or portions of limbs, common bones being sold as relics of the saints. The men who do this are plainly such of whom St Paul speaks, that they suppose gain to be godliness; for they make into a mere excrement of their money-bags the things which (if they but knew it) would tend to the salvation of their souls.

As you can tell from the last example, however, relics believed to be the bonefide (pun) thing were frequently bought, stolen, traded, lost, etc. About one hundred years later, Louis IX purchased the crown of thorns for his royal chapel Ste.-Chapelle for a whopping 135,000 livre ~1240, half the annual budget (the chapel, in comparison, cost 60,000 livre. The French king had purchased the relics from Baldwin II, emperor of Latin Constantinople, by way of the Venetians, to whom Baldwin had pawned the crown. At the end of the day, it all depends on who's selling, who's buying, and the quality of the relic involved (A-List relics like the crown of thorns are big money). For example, in 1411, the anti-pope John XXII decided to auction off the head of John the Baptist, the price set at 50,000 florins, and managed to enter into negotiations with a delegation from Florence. Talks however, reached a standstill, and the Florentines walked away with only one of the saint's fingers. If you had the power and position to be dealing in well-acclaimed relics, the sky was pretty much the limit, but at the end of the day, relic selling is tricky business, wrought with forgeries, double dealings, plotting, and no small measure of suspicion. Seller beware.

Or you could just pull a Venetian and smuggle the relics out in a pork barrel to dupe the Muslim guards of Alexandria (so the tradition goes).

You would make more by charging admission for pilgrims to see it.