Impossible to say, but I guess you would have to work out fuel consumption for the particular vehicle that they are using and pay a proportion of that, and some on top for the driver's time and effort, unless you are able to share transport with another or more, that would keep the costs down.
1	Impossible	impossible	ADJ	JJ	Degree=Pos	0	root	0:root	_
2	to	to	PART	TO	_	3	mark	3:mark	_
3	say	say	VERB	VB	VerbForm=Inf	1	csubj	1:csubj	SpaceAfter=No
4	,	,	PUNCT	,	_	7	punct	7:punct	_
5	but	but	CCONJ	CC	_	7	cc	7:cc	_
6	I	I	PRON	PRP	Case=Nom|Number=Sing|Person=1|PronType=Prs	7	nsubj	7:nsubj	_
7	guess	guess	VERB	VBP	Mood=Ind|Number=Sing|Person=1|Tense=Pres|VerbForm=Fin	1	conj	1:conj:but	_
8	you	you	PRON	PRP	Case=Nom|Person=2|PronType=Prs	10	nsubj	10:nsubj|12:nsubj:xsubj|25:nsubj:xsubj	_
9	would	would	AUX	MD	VerbForm=Fin	10	aux	10:aux	_
10	have	have	VERB	VB	VerbForm=Inf	7	ccomp	7:ccomp	_
11	to	to	PART	TO	_	12	mark	12:mark	_
12	work	work	VERB	VB	VerbForm=Inf	10	xcomp	10:xcomp	_
13	out	out	ADP	RP	_	12	compound:prt	12:compound:prt	_
14	fuel	fuel	NOUN	NN	Number=Sing	15	compound	15:compound	_
15	consumption	consumption	NOUN	NN	Number=Sing	12	obj	12:obj	_
16	for	for	ADP	IN	_	19	case	19:case	_
17	the	the	DET	DT	Definite=Def|PronType=Art	19	det	19:det	_
18	particular	particular	ADJ	JJ	Degree=Pos	19	amod	19:amod	_
19	vehicle	vehicle	NOUN	NN	Number=Sing	15	nmod	15:nmod:for|23:obj	_
20	that	that	PRON	WDT	PronType=Rel	23	obj	19:ref	_
21	they	they	PRON	PRP	Case=Nom|Number=Plur|Person=3|PronType=Prs	23	nsubj	23:nsubj	_
22	are	be	AUX	VBP	Mood=Ind|Number=Plur|Person=3|Tense=Pres|VerbForm=Fin	23	aux	23:aux	_
23	using	use	VERB	VBG	Tense=Pres|VerbForm=Part	19	acl:relcl	19:acl:relcl	_
24	and	and	CCONJ	CC	_	25	cc	25:cc	_
25	pay	pay	VERB	VB	VerbForm=Inf	12	conj	10:xcomp|12:conj:and	_
26	a	a	DET	DT	Definite=Ind|PronType=Art	27	det	27:det	_
27	proportion	proportion	NOUN	NN	Number=Sing	25	obj	25:obj	_
28	of	of	ADP	IN	_	29	case	29:case	_
29	that	that	PRON	DT	Number=Sing|PronType=Dem	27	nmod	27:nmod:of	SpaceAfter=No
30	,	,	PUNCT	,	_	32	punct	32:punct	_
31	and	and	CCONJ	CC	_	32	cc	32:cc	_
32	some	some	DET	DT	_	27	conj	25:obj|27:conj:and	_
33	on	on	ADP	IN	_	34	case	34:case	_
34	top	top	NOUN	NN	Number=Sing	32	nmod	32:nmod:on	_
35	for	for	ADP	IN	_	39	case	39:case	_
36	the	the	DET	DT	Definite=Def|PronType=Art	37	det	37:det	_
37-38	driver's	_	_	_	_	_	_	_	_
37	driver	driver	NOUN	NN	Number=Sing	39	nmod:poss	39:nmod:poss	_
38	's	's	PART	POS	_	37	case	37:case	_
39	time	time	NOUN	NN	Number=Sing	25	obl	25:obl:for	_
40	and	and	CCONJ	CC	_	41	cc	41:cc	_
41	effort	effort	NOUN	NN	Number=Sing	39	conj	25:obl:for|39:conj:and	SpaceAfter=No
42	,	,	PUNCT	,	_	25	punct	25:punct	_
43	unless	unless	SCONJ	IN	_	46	mark	46:mark	_
44	you	you	PRON	PRP	Case=Nom|Person=2|PronType=Prs	46	nsubj	46:nsubj|48:nsubj:xsubj	_
45	are	be	AUX	VBP	Mood=Ind|Number=Sing|Person=2|Tense=Pres|VerbForm=Fin	46	cop	46:cop	_
46	able	able	ADJ	JJ	Degree=Pos	25	advcl	25:advcl:unless	_
47	to	to	PART	TO	_	48	mark	48:mark	_
48	share	share	VERB	VB	VerbForm=Inf	46	xcomp	46:xcomp	_
49	transport	transport	NOUN	NN	Number=Sing	48	obj	48:obj	_
50	with	with	ADP	IN	_	51	case	51:case	_
51	another	another	DET	DT	_	48	obl	48:obl:with	_
52	or	or	CCONJ	CC	_	53	cc	53:cc	_
53	more	more	ADJ	JJR	Degree=Cmp	51	conj	48:obl:with|51:conj:or	SpaceAfter=No
54	,	,	PUNCT	,	_	46	punct	46:punct	_
55	that	that	PRON	DT	Number=Sing|PronType=Dem	57	nsubj	57:nsubj	_
56	would	would	AUX	MD	VerbForm=Fin	57	aux	57:aux	_
57	keep	keep	VERB	VB	VerbForm=Inf	7	parataxis	7:parataxis	_
58	the	the	DET	DT	Definite=Def|PronType=Art	59	det	59:det	_
59	costs	cost	NOUN	NNS	Number=Plur	57	obj	57:obj	_
60	down	down	ADV	RB	_	57	advmod	57:advmod	SpaceAfter=No
61	.	.	PUNCT	.	_	1	punct	1:punct	_
