Well, as devastating and as knock-down of an argument as that was...n't, I do feel the need to tap on RG Price's point a bit, but ultimately I'll conclude that its pretty sound.
Surely, the original *authors* of the original texts didn't get their ideas from other texts. Rather, they had some pre-existing ideas, which they wrote in order to express as the best they could....but still pretty badly. E.g. Paul tells us vociferously that he got his ideas from direct revelation. But he did such a bad job of explaining it that generations of later writers felt the need to recontextualize and rewrite him to the point that his original vision is very obscured.
Which brings us to Mark. I've been persuaded that Mark was writing in service of rationalizing Paul's letters, but even he is writing at enough distance from Paul that he doesn't fully know what to make of him--he's just as confused about some very basic points of Paul's theology (e.g. how seriously should take the law of Moses?) as we are today about Paul. And he adds a metric truckload of his own secrets and mysteries.
I find Mark all but incomprehensible, but I'm in good company--Matthew completely missed the whole Mark as allegory thing. He recognized the allusions to the Torah, but mistook them as fulfilled prophecy. He even larded his gospel with yet more proof-texty citations from all over the place. Some of them are really strained, and some of them we can't even find in any copy of the Torah we have today--god only knows where he got them, but they all were "it was written" in some text or another.
The various Lukes probably had the best understanding of Mark, but canonical Luke-Acts are transparently anti-Marcionite texts, designed explicitly to get us to view Mark and Paul through proto-orthodox goggles. Once again, whatever the original ideas were, they were being overwritten, reinterpreted, and their original prominence made so obscure that all we have is the text, standing in the way of seeing the original ideas.
By the time we get to John, Mark is obliterated completely, and the author doesn't even refer to the Torah to clue us in, we get this Jesu-Logos, from Philo. I.e. from another written text.
At every stage, we can see how they misread the texts and got nutty ideas, and how they muddied the waters even further with new nutty ideas of their own--but those nutty ideas came by interpretation of source texts. And whoever first published the 4-fold gospel collection *deliberately* remained completely silent about when and where they were written, and how their original authors read them. Even the names they gave them are a transparent attempt to invoke apostolic authority while hiding their true provenance.
Back to RG Price's thesis that all these ideas came from texts. Since the original ideas of the original authors has so totally been hidden from us, where *else* could these ideas come from but the texts which mysteriously reached us? Really, there's only one more consideration preventing us from full-throated agreement, and even that falls away under pressure, viz, that any of the gospels, even the canonical gospels, can--via judiciously straining of gnats and swallowing of camels--be interpreted as supporting any version of Christianity, ancient or modern. The docitist loved Canonical Luke, the gnostics loved John--and even today there are arians (Jehovah's Witnesses, Unitarians) who thump the same Bible as the Trinitarians do.
The church fathers recognized this, and their answer was lame, but effective: *we*, they say, descend in an unbroken chain from the Apostles, so we are the ones who know how to interpret them. It's pretty ironic, because the only *actual* chain of apostolic authority that they give is for Marcion--they all say he was a student of Cerdo, who was a student of Simon Magus, who was a student of Philip the Apostle. Maybe they just made that up to discredit him, but I take it semi-seriously, because, after all, any of them could have made up their own chain of Apostolic descent, but didn't think they could get away with it. If they had even tried, they would have been exposed as fraud, because the chain was looong broken by the time they showed up.
Point being, all almost anybody had, at any stage of the process, was a mysterious set of texts, which they didn't really have any idea who wrote them or where they came from or what they meant. By the time we came around, the origins are so obscure that it is a viable intellectual position to hold that there was no Jesus at all, just a lot of interesting stories. It may be true or false, but the fact remains, any history we have is so obscure that, I have to concur, the only real place anybody ever could have gotten these ideas is from a written text. And anybody who ever seriously wrote about these texts--ancient or modern, theologians or scholars--makes the same claim: they get their ideas from the texts, they aren't just making this stuff up.