reference - demonstrative determiner

I bought a pen and a pencil yesterday. What is surprising is this is more expensive than that.

In this sentence, I am not sure what "this" and "that" are referring to. I first thought that "this" refers to "pencil" and "that" refers to any pen that is distant from the speaker.

How can I infer what 'this' and 'that' represent here? Can the represented element of both be inferred from what precedes, ie "a pen and pencil"?

It is not the usual way to say that; as far as I know it should not be said that way. In formal writing this would be said as follows.

• I bought a pen and a pencil yesterday. What is surprising is that the latter is more expensive than the former.

In a colloquial context you could say this.

• I bought a pen and a pencil yesterday. What is surprising is that the pencil is more expensive.

• I bought a pen and a pencil yesterday. Surprisingly, the pencil is more expensive.

Addition due to a comment from user 405662

It American English "this" stands for the closest item in the line and that for the other, therefore "this" stands for "pencil" and "that" for "pen" (Merriam-Webster). I do not know whether this is current usage in British English.
For instance, I find the following in the SOED.

3. a In opposition to that; the first of two or more things, esp. the nearer or more immediate or obvious, the thing actually at hand. b [Latinism] The latter now rare or obsolete Middle English.