Allies, Contacts, and Patrons are a useful part of GURPS, representing a significant social advantage often ignored by other game systems. Below is a partial cost summary and breakdown, for full details you need to read GURPS Basic: Characters.
Contacts are rated by their effective skill level in a specific skill. Wildcard Contacts make this an entire skill group, typically by profession - Business, military, police, etc.
A Contact will answer a question or do something for the character as long as it is at least two of nonhazardous, inexpensive (or mostly paid for), or quick.Contact -12  Contact Group -12 
Contact -15  Contact Group -15 
Contact -18  Contact Group -18 
Contact -21  Contact Group -21 
The number after Contact reflects effective skill (Contact -12 means skill 12).
It is important to note that this is effective skill and comes not just from points in a skill but access to lands, libraries, workshops, even assistants. So high skill levels are not unreasonable or cinematic.
Ally and Ally Group
Allies are based on a relevant percentage of the character. This means they are more efficient and powerful the more points the character is based on. They also are willing to take risks and go adventuring! Whereas Contacts are asked one task per session with additional tasks acquiring a cumulative penalty. Players will typically choose Allies over Contacts.
Allies 25%  Ally Group 6-10  Ally Group 11-20 
Allies 50%  Ally Group 6-10  Ally Group 11-20 
Allies 75%  Ally Group 6-10  Ally Group 11-20 
Allies 100%  Ally Group 6-10  Ally Group 11-20 
So Contact -12 and Ally 25% cost the same and are pretty equivalent in most campaigns.
Contact -15 and Ally 50% cost the same, also likely equivalant in most campaigns though highly cinematic and especially supers campaigns the Ally is going to have higher skills than 15.
Contact -18 and Ally 75% cost the same. At this level the Contact likely has a superior skill level in the main skill even in a four color campaign. The Ally will however usually have more breadth and a full character sheet.
Which is Better?
.Ally seems better, you get to ask for help as often as you like and they are willing to take great risks for you.
However the part people often overlook is that the Contacts skill represents facilities and resources not on a chartecter sheet. That Forensics-12 skill represents a lab worker in a forensics lab, while Forensics -18 or -21 likely represents a supervisor or manage who van assign people to get the analysis done.
The Ally doesn't come with a lab so one has to be provided for by him or the player character.
And if it was a Contact Group that likely means most of the labs team are willing to work for you and you can get several projects worked on at once or ask for follow-ups without having to wait.
Contacts can aslo do field work or research, Streetwise to ask around for you. Batman, Daredevil, and other crime fighters have Streetwise burt also Contacts with Streetwise. So why buy a Contact for a skill you already have? Because if they do the offscreen legwork it saves gametime (meaning GMs should encourage Contacts, even as free point awards) and lets the pc do something perhaps more interesting. Especially helful for groups of players so they are not sitting around while the PC runs around busting bad guys chops or something and saves the GMs creativity for other tasks.
Contact (Administration -12) could work at the DMV or police station and maybe run a plate for the private investigator. This probably counts as hazardous as they risk losing their job if caught.
If that Contact worked at city hall they could likely pull city records instead.
Patrons represent very powerful individuals or organizations, even governments. They wont go on adventures with you but may send along some guys for a mission. They also can provide money and equipment or other resources. GURPS Boardroom and Curia is a great resource for Patrons.
What About Patrons?
GURPS Social Engineering: Keeping in Contact goes into more details on Contacts.
GURPS Boardroom and Curia as mentioned covers Patrons and Tomorrows Ride's is an example Patron that furnishes action heroes fancy cars.
Modular Abilities allows multiple Contacts, so purchasing Contacts as alternative abilities seem reasonable and is suggested in a few places. The main limit is a failed appearance roll disables the whole thing - including any alternate Contacts!
Patrons provide what is reasonable from the organization in question. Take Contact or Contact Group as an alternative ability for people within that organization. This represents a bigger investment but offers alternate and specific ways to get assistance from that organization.
Ally Groups could represent reinforcements or a combat team
GURPS Powers: Divine Favor deals with having God as your Patron.
GURPS Social Engineering: Pulling Rank is another way of handling Patron and Ranks.