Something is of/in the order of

In the UK, the idiom is of the order of (at least, in the example quoted, "of the order of 100kg").

In [the] order of specifies a sequence; and on the order of specifies who made an edict.

ODO entry:

of the order of: approximately; having the order of magnitude specified by...

The vast majority of these citations from Google Books will probably all match OP's context...

...distance is of the order of... 6670 results

...distance is in the order of... 1050 results

...distance is on the order of... 907 results

Whilst I don't see clear-cut reasons for saying any particular preposition is either "right" or "wrong", I personally prefer in. The repetition of of slightly turns me off the first alternative, and I simply don't come across on very often, so it seems a little "strange" to my ear.

I would also just say the ratios implied by figures are only an approximate guide to actual usage. The corresponding figures for "weight" are of:2680, in:1150 on:396. But the basic message is they're all used reasonably often, but of is the most common form.