On average, cow/bull skulls sell from 15-40 USD in shops and markets. It really depends on who you buy it from and what it looks like. For a good price, go to a regular street market, not a boutique shop. If it has its horns, it will be considerably more. Even the nicest ones around don't go for more than 40 USD.
Why must you go to Nogales to do this deal?
Product, there are people here with all kinds of opinions about various places. Some enjoy Nogales, some hate it and are not afraid to say so. Go where your interests draw you. Personally I find the town to be a fun day of shopping and eating.
If you do decide to go, just remember you are walking into a very poor area. Nobody told me this the first time I went, and I walked in like a silly, uneducated american wearing my fancy brand name clothes, real jewelry and carrying a very nice brand name bag. I felt horrible. If ever I went again, I would bring coins or other small trinkets to give to the children that ask. I was very unprepared and I think that ruined the experience for me.
Nogales is a border town, and is not at all like the colonial cities. It is a very prosperous business and industrial city, but that doesn't translate into the tourist area. The tourist area near the border is far less than it was in the past. Many of the restaurants and shops have closed as a result of violence associated with cartels and drugs, and avoidance by tourists.
You can buy skulls at roadside vendors in the Tucson area.
If you do decide to go to Nogales, park on the US side and walk across the border. If you take I-19 south, it ends within a few blocks of the border. At the end of I-19, just after it becomes a city street and after West St, you will find a large parking lot right at the border on the right.
As I said, some people love it and some hate it. If you want to see some of the history check out the church just a couple of blocks south of the entry gate. I like it a lot.
There are many things you can get down there that you won't find in the "Mexican shops" north of the border. I find the hand made silver and semi-precious stone jewelry to be a special bargain. Antiques are also a good deal, but you have to dig for them. If you want a good time at bargain prices, check out the links below.
....thank you for your assistance!