1 way ticket from Nanka Namba to Koyasan

I'm planning to go Koyasan from Namba, as I only going ONE way (not returning to Namba), can I purchase one way tickets instead of getting the Koyasan World Heritage Ticket? A check on Hyperdia shows the fare to be 1,260 yen taking the Limited Express & Cable-Car. Within Koyasan, I expect to take only 2 bus rides, not worth to get a day Bus Pass. Is possible to pay cash on bus? Thanks.

Do you mean you will stay in Koyasan and not going elsewhere?

For me, I will buy the Ticket at Shin Imamiya station, take the Nankai Koya Line or Nankai Koya Line Exp (I dont think this is the LIMITED EXP as this is reserved seats while the other two aforementioned are non-reserved) and cable car to reach Koyasan.
From Koyasan, there is a part of journey (though about 2km) accessible only by bus before one can reach the city area.
The return journey is similar.
After working out the cost, the Ticket seems cheaper.

I�fm only going ONE way (not returning to Namba)
Within Koyasan, I expect to take only 2 bus rides

To which station of Nankai will you travel back?

With a Koyasan World Heritage Ticket which is valid from Namba Station,
you can start your Mount Koya trip from Namba Station and end it on the next day in another station in Osaka City, abandoning the rest of your ride to Namba Station.

The price of the regular version of a Koyasan World Heritage Ticket valid from Namba Station is 2860 yen.
A Nankai Rinkai Bus fare for one section from/to Koyasan Station bus stop is at least 220 yen; for other sections, a bus fare per section is at least 140 yen.

If you start the trip from Namba Station and end it on the next day in Tengachaya Station, for example,
the one-way railroad + cable car fares are 1260 yen for each of the two ways, and the sum is 2520 yen;
2520 yen + 220 yen + 140 yen = 2860 yen;
so, unless you can avoid using Koyasan Station bus stop, a Koyasan World Heritage Ticket is likely to pay off.
(And, you might be given a discount by using a coupon attached to the pass at a certain historical facility or souvenir shop.)

the fare to be 1,260 yen taking the Limited Express & Cable-Car.

When you travel with regular Nankai tickets and take a Limited Express train from Namba to Gokurakubashi, you need to pay 780 yen as a charge on the Limited Express service, in addition to 1260 yen as a basic fare.
(The Limited Express Version of a Koyasan World Heritage Ticket can cover the both.)

I think the more expensive version of the ticket covers only one way (to Koyasan) on LIMITED EXPRESS:
http://www.japan-guide.com/e/e2358_012.html
See item 3 under Validity.

Thank you @Starlight & @Omotenashi for the information. I will actually be travelling from Kyoto to Namba to take the train to Koyasan. It will be One Way up to Koyasan as I am travelling down to Ryujin on the same day. The Koyasan World Heritage Ticket actually covers 2-way fare.

I didn't know that I need top up for the Limited Express by getting the one way ticket. I do not mind a regular train, but I afraid that it may not be available.

For my two bus rides will be from Koyasan Station to Central Information Office to deposit my backpack at coin locker, then walk to Okunoin. Thereafter, come back out to Okunoin-Mae bus to Kongo Buji-Mae stop to visit Garan. @Starlight mention first leg bus is 220Yen. Any idea how much will my 2nd ride cost?

Hi
I have not been to Koyasan and thus unsure the details.
Maybe you can read
http://www.nankaikoya.jp/en/stations/ticket.html

can I purchase one way tickets instead of getting the Koyasan World Heritage Ticket?
YES.
Is possible to pay cash on bus?
of course, YES.